IP v4 addresses are 32 bit extended

IPv4 addresses are 32 bits extended; like all network-level addresses, they've a network portion plus a host portion. The network portion uniquely identifies a physical or logical link and is widespread to all devices attached to that link. The host portion uniquely identifies a particular device attached for the link.

There are numerous ways to represent the 32 bits of an IP address. For instance, the 32-bit IP address

00001010110101100101011110000011

might be represented in decimal as

181,819,267.

The binary format is cumbersome, plus a decimal format of the complete 32-bit number is time-consuming to calculate. Figure 1-4 shows a improved format.

Figure 1-4. The dotted-decimal format can be a hassle-free approach to write IPv4 addresses, nevertheless it shouldn't be confused with what the router (or host) sees: a 32-bit string.

image

The 32 bits of the address comprise four octets, each and every of which might be represented using a decimal number between 0 and 255, with dots between the decimal representations. In Figure 1-4, the 32-bit address is mapped into a dotted-decimal representation.8

8 Dotted decimal is utilised only with IPv6 addresses. As you'll read in Chapter 2, IPv6 addresses are represented entirely differently.

An vital distinction to don't forget when working with IPv4 addresses is the fact that dotted decimal is just a simple way for humans to read and write IP addresses. Normally don't forget that the router will not be reading an address when it comes to four octets; rather, the router sees a 32-bit binary string. Many pitfalls might be avoided by keeping this fact firmly in thoughts. If you have not worked with binary numbersparticularly converting between binary and decimalyou may well want to read the tutorial in Appendix A, "Tutorial: Operating with Binary and Hex," prior to continuing on with this chapter.

Possibly essentially the most distinctive characteristic of IPv4 addresses is the fact that in contrast to other network-level addresses, the network and host portions can differ in size inside the 32-bit boundaries. That is certainly, the network portion may well take up a lot of the 32 bits, or the host portion may well, or they may well divide the bits equally. Protocols for instance NetWare and AppleTalk had been developed for use in reasonably tiny networks, and consequently their network-level addresses have fixed-length network and host portions. This arrangement absolutely makes life simpler; a receiving device knows to read a specific quantity of bits in to the address to find the network part, plus the rest is host address.

TCP/IP, even so, was developed from the initial to be flexible enough to be utilised in any network, from the tiny for the colossal. This flexibility makes IP addresses much more difficult to manage. The fundamentals of administering IP addresses are presented in this section, after which some much more advanced techniques are introduced in Chapter 6, "RIPv2, RIPng, and Classless Routing."

Initially Octet Rule

Devoid of placing as well fine a point on it, it could be stated that you will find 3 sizes of networks as measured by the quantity of hosts: big, medium, and tiny:

Huge networks, by definition, have a enormous quantity of hosts. Reasonably few big networks exist.
Compact networks are just the opposite. Each a single is tiny due to the fact it features a tiny quantity of hosts; a massive quantity of tiny networks exist.
Medium networks are just that: a medium quantity of them (in relation to big and tiny ones) plus a medium quantity of hosts in each and every a single.

This high degree of addressing focus calls for 3 typesclassesof network address for the 3 sizes of networks. Addresses for big networks ought to be capable of addressing several hosts, but due to the fact so few big networks exist, only a few big-network addresses are required.

The situation is reversed for tiny networks. Mainly because there are various tiny networks, a sizable quantity of small-network addresses are required. But due to the fact a tiny network features a tiny quantity of hosts, each and every of the several network addresses only calls for a few host addresses.

For medium-sized networks, a medium quantity of network addresses plus a medium quantity of host addresses are going to be obtainable for each and every network address.

Figure 1-5 shows how the network and host portions of IPv4 addresses are divided up for these 3 classes.

Figure 1-5. Class A, B, and C IPv4 address formats.

The big, medium, and tiny networks described therefore far map to address classes as follows:

Class A IPv4 addresses are for big networks. The initial octet could be the network portion, plus the last 3 octets are the host portion. Only 256 numbers are obtainable within the eight-bit network part, but 224 or 16,777,216 numbers are obtainable within the host part of each and every of these network addresses.
Class B addresses are for medium-size networks. The initial two octets are the network portion, plus the last two octets are the host portion. You will discover 216 or 65,536 obtainable numbers within the network part and an equal number within the host part.
Class C addresses are just the opposite of Class A. The initial 3 octets are the network portion, plus the last octet could be the host portion.

Mainly because all IPv4 addresses are 32-bit binary strings, a way of distinguishing the class to which a particular address belongs is important. The initial octet rule, demonstrated in Table 1-3, supplies the indicates to make such a distinction and may be described as follows:

For Class A addresses, the initial bit of the very first octetthat is, the left-most bit of the complete 32-bit stringis often set to zero. For that reason, we are able to uncover the minimum and maximum numbers within the Class A range by setting all of the remaining bits within the very first octet to zero (for the minimum) and a single (for the maximum). This action outcomes within the decimal numbers 0 and 127 using a few exceptions: 0 is reserved as part of the default address (Chapter 12, "Default Routes and On-Demand Routing"), and 127 is reserved for internal loopback addresses.9 That leaves 1 through 126; any IP address whose very first octet is between 1 and 126 inclusive can be a Class A address.

9 Devices use loopback addresses (typically 127.0.0.1) to send targeted traffic to themselves. Information might be sent to this address and returned for the transmitting process without having ever leaving the device.

Class B addresses often have their left-most bit set to a single plus the second bit set to zero. Once more, locating the minimum and maximum quantity of the initial octet by setting all remaining bits to zero after which to a single, you see in Figure 1-4 that any address whose very first octet is within the decimal range 128 through 191 can be a Class B address.
In Class C addresses, the initial two bits are set to a single, plus the third bit is set to zero. The outcome can be a very first octet range of 192 through 223.10

10 Notice that 223 will not exhaust all obtainable numbers within the very first octet. See Configuration Exercising 1 in the finish of this chapter.

Table 1-3. Initially octet rule.

Rule

Minimum and Maximum

Decimal Range

Class A: Initially bit is often 0

00000000 = 0

01111111 = 127

1126*

Class B: Initially two bits are often ten

10000000 = 128

10111111 = 191

128191

Class C: Initially 3 bits are often 110

11000000 = 192

11011111 = 223

192223

* 0 and 127 are reserved

So far IPv4 addressing does not seem so difficult. A router or host could effortlessly ascertain the network part of an IP address by applying the initial octet rule. If the very first bit is 0, then read the initial eight bits to find the network address. If the very first two bits are ten, then read the initial 16 bits; and when the very first 3 bits are 110, then read 24 bits in to obtain the network address. However, things aren't that straightforward.

Address Masks

The address for an complete data linka non-host-specific network addressis represented by the network portion of an IP address, with all host bits set to zero. For instance, an addressing authority11 may well assign to an applicant an address of 172.21.0.0.12 This address can be a Class B address due to the fact 172 is between 128 and 191, so the last two octets make up the host bits. Notice that they are all set to zero. The initial 16 bits (172.21.) are assigned, but address owners are free of charge to do whatever they please with the host bits.

11 The high-level organizations accountable for managing and assigning IP addresses are APNIC in Asia, ARIN in North America, LACNIC in Central and South America, and RIPE in EMEA.

12 Truly, this address would never ever be assigned. It is from a group of addresses reserved for private use; a lot of the addresses utilised in this book are from this reserved pool, described in RFC 1918. Reserved addresses are ten.0.0.010.255.255.255, 172.16.0.0172.31.255.255, and 192.168.0.0192.168.255.255.

Each device or interface are going to be assigned a unique, host-specific address for instance 172.21.35.17. The device, regardless of whether a host or a router, obviously requires to understand its personal address, nevertheless it also requires to be in a position to ascertain the network to which it belongsin this case, 172.21.0.0.

This job is accomplished by indicates of an address mask. The address mask can be a 32-bit string, a single bit for each and every bit of the IPv4 address. As a 32-bit string, the mask might be represented in dotted-decimal format just like an IPv4 address. This representation tends to be a stumbling block for some newbies: Even though the address mask might be written in dotted decimal, it is not an address. Table 1-4 shows the typical address masks for the 3 classes of IPv4 address.

Table 1-4. Address masks for Class A, B, and C IPv4 addresses.

Class

Mask

Dotted Decimal

A

11111111000000000000000000000000

255.0.0.0

B

11111111111111110000000000000000

255.255.0.0

C

11111111111111111111111100000000

255.255.255.0

For each and every bit of the IPv4 address, the device performs a Boolean (logical) AND function with the corresponding bit of the address mask. The AND function might be stated as follows:

Compare two bits and derive a outcome. The outcome are going to be a single, if and only if, each bits are a single. If either or each bits are zero, the outcome are going to be zero.

Figure 1-6 shows how, to get a given IPv4 address, the address mask is utilised to ascertain the network address. The mask features a a single in each and every bit position corresponding to a network bit of the address plus a zero in each and every bit position corresponding to a host bit. Mainly because 172.21.35.17 can be a Class B address, the mask will need to have the initial two octets set to all ones plus the last two octets, the host part, set to all zeros. As Table 1-4 shows, this mask might be represented in dotted decimal as 255.255.0.0.

Figure 1-6. Each bit of this Class B address is ANDed with the corresponding bit of the address mask to derive the network address.

image

A logical AND is performed on the IPv4 address and its mask for each and every bit position; the outcome is shown in Figure 1-6. Inside the outcome, each and every network bit is repeated, and all of the host bits grow to be 0s. So by assigning an address of 172.21.35.17 plus a mask of 255.255.0.0 to an interface, the device will know that the interface belongs to network 172.21.0.0. Applying the AND operator to an IPv4 address and its address mask often reveals the network address.

An address and mask are assigned to an interface of a Cisco router (in this instance, the E0 interface) by indicates of the following commands:

Smokey(config)# interface ethernet 0
Smokey(config-if)# ip address 172.21.35.17 255.255.0.0

But why use address masks at all? So far, applying the initial octet rule seems considerably easier.

Subnets and Subnet Masks

Never ever lose sight of why network-level addresses are important within the very first place. For routing to be accomplished, each and every and each and every data link (network) will need to have a unique address; moreover, each and every and each and every host on that data link will need to have an address that each identifies it as a member of the network and distinguishes it from any other host on that network.

As defined so far, a single Class A, B, or C address might be utilised only on a single data link. To build a network, separate addresses must be utilised for each and every data link in order that these networks are uniquely identifiable. If a separate Class A, B, or C address had been assigned to each and every data link, fewer than 17 million data links may be addressed prior to all IPv4 addresses had been depleted. This approach is obviously impractical,13 as could be the fact that to make complete use of the host address space within the previous instance, greater than 65,000 devices would must reside on data link 172.21.0.0!

13 Seventeen million data links may well seem like a whole lot until you contemplate that even a single moderate-size enterprise may well have dozens or hundreds of data links.

The only approach to make Class A, B, or C addresses practical is by dividing each and every key address, for instance 172.21.0.0, into subnetwork addresses. Recall two details:

The host portion of an IPv4 address might be utilised as desired.
The network portion of an IPv4 address is determined by the address mask assigned to that interface.

Figure 1-7 shows a network to which the key Class B address 172.21.0.0 has been assigned. 5 data links are interconnecting the hosts and routers, each and every a single of which calls for a network address. Because it stands, 172.21.0.0 would need to be assigned to a single data link, after which four much more addresses would need to be requested for the other four data links.

Figure 1-7. Subnet masks allow a single network address to be utilised on many data links by "borrowing" a few of the host bits for use as subnet bits.

image

Notice what was performed in Figure 1-7. The address mask will not be a typical 16-bit mask for Class B addresses; the mask has been extended another eight bits in order that the initial 24 bits of the IP address are interpreted as network bits. In other words, the routers and hosts have been given a mask that causes them to read the initial eight host bits as part of the network address. The outcome is the fact that the key network address applies for the complete network, and each and every data link has grow to be a subnetwork, or subnet. A subnet can be a subset of a major Class A, B, or C address space.

The IPv4 address now has 3 components: the network part, the subnet part, plus the host part. The address mask is now a subnet mask, or a mask that's longer than the typical address mask. The initial two octets of the address will often be 172.21, but the third octetwhose bits are now subnet bits rather of host bitsmight range from 0 to 255. The network in Figure 1-6 has subnets 1, 2, 3, four, and 5 (172.21.1.0 through 172.21.5.0). Up to 256 subnets might be assigned beneath the single Class B address, applying the mask shown.

Two words of caution are so as. Initially, not all routing protocols can assistance subnet addresses in which the subnet bits are all zeros or all ones. The reason is the fact that these protocols, named classful protocols, cannot differentiate between an all-zero subnet plus the key network number. For instance, subnet 0 in Figure 1-7 would be 172.21.0.0; the key IP address is also 172.21.0.0. The two cannot be distinguished without having further facts.

Likewise, classful routing protocols cannot differentiate a broadcast on the all-ones subnet from an all-subnets broadcast address.14 For example, the all-ones subnet in Figure 1-7 would be 172.21.255.0. For that subnet, the all-hosts broadcast address would be 172.21.255.255, but that's also the broadcast for all hosts on all subnets of key network 172.21.0.0. Once more, the two addresses cannot be distinguished without having further facts. RIP version 1 and IGRP are each classful routing protocols; Chapter 7, "Enhanced Interior Gateway Routing Protocol (EIGRP)," introduces classless routing protocols, which can indeed use the all-zeros and all-ones subnets.

14 The all-hosts IP broadcast address is all ones: 255.255.255.255. An all-hosts broadcast to get a certain subnet would set all host bits to a single; for instance, an all-hosts broadcast for subnet 172.21.1.0 would be 172.21.1.255. Lastly, a broadcast for all hosts on all subnets sets the subnet bits plus the host bits to all ones: 172.21.255.255.

The second caution has to do with the verbal description of subnets and their masks. Subnetting the third octet of a Class B address, as is performed is Figure 1-7, is pretty widespread; also widespread is hearing folks describe such a subnet design as "using a Class C mask using a Class B address," or "subnetting a Class B address into a Class C." Both descriptions are incorrect! Such descriptions frequently lead to misunderstandings about the subnet design or to a poor understanding of subnetting itself. The correct approach to describe the subnetting scheme of Figure 1-6 is either as "a Class B address with 8 bits of subnetting," or as "a Class B address using a 24-bit mask."

The subnet mask might be represented in any of the following 3 formats:

Dotted decimal: 255.255.255.0

Bitcount: 172.21.0.0/24

Hexadecimal: 0xFFFFFF00

Dotted decimal is commonly utilised in computer software which has been around to get a when, despite the fact that the bitcount format is becoming increasingly preferred. In comparison with dotted decimal, the bitcount format is simpler to write. (The address is followed by a forward slash plus the quantity of bits which can be masked for the network part.) In addition, the bitcount format is much more descriptive of what the mask is definitely performing and for that reason avoids the type of semantic misunderstandings described within the previous paragraph. Some UNIX systems use the hexadecimal format.

Even though the address mask must be specified to Cisco routers in dotted decimal, applying the command shown previously, the mask might be displayed by several show commands in any of the 3 formats by applying the command ip netmask-format decimal in line configuration mode. For example, to configure a router to display its masks in bitcount format, use

Gladys(config)# line vty 0 four
Gladys(config-line)# ip netmask-format bit-count

Designing Subnets

As established within the previous section, subnet bits cannot be all zeros or all ones in classful environments. Likewise, an IPv4 host address cannot have all its host bits set to zerothis setting is reserved for the address that routers use to represent the network or subnet itself. Along with the host bits cannot be set to all ones, as this setting could be the broadcast address. These restrictions apply for the host bits with no exceptions and are starting points for designing subnets. Beyond these starting points, network designers ought to pick out essentially the most appropriate subnetting scheme when it comes to matching the address space for the particulars of a network.

When designing subnets and their masks, the quantity of obtainable subnets beneath a major network address plus the quantity of obtainable hosts on each and every subnet are each calculated with the same formula: 2n 2, exactly where n could be the quantity of bits within the subnet or host space and 2 is subtracted to account for the unavailable all-zeros and all-ones addresses. For example, given a Class A address of ten.0.0.0, a subnet mask of ten.0.0.0/16 (255.255.0.0) indicates that the 8-bit subnet space will yield 28 2 = 254 obtainable subnets and 216 2 = 65,534 host addresses obtainable on each and every of these subnets. However, a mask of ten.0.0.0/24 (255.255.255.0) indicates that a 16-bit subnet space is yielding 65,534 subnets and an 8-bit host space is yielding 254 host addresses for each and every subnet.

The following methods are utilised to subnet an IPv4 address:

Step 1.

Determine how several subnets are required and how several hosts per subnet are required.

Step 2.

Use the 2n 2 formula to ascertain the quantity of subnet bits plus the quantity of host bits that can satisfy the requirements established in Step 1. If many subnet masks can satisfy the requirements, pick out the a single that can finest scale to future requires. For example, when the network is probably to develop by adding subnets, pick out much more subnet bits; when the network is probably to develop by adding hosts to existing subnets, pick out much more host bits. Keep away from deciding on a scheme in which either all subnets or all host addresses inside the subnets are going to be utilised up instantly, leaving no space for future growth.

Step 3.

Operating in binary, ascertain all obtainable bit combinations within the subnet space; in each and every instance, set all of the host bits to zero. Convert the resulting subnet addresses to dotted decimal. These are the subnet addresses.

Step four.

For each and every subnet address, once more working in binary, write all potential bit combinations for the host space without having changing the subnet bits. Convert the outcomes to dotted decimal; these are the host addresses obtainable for each and every subnet.

The value of performing the last two methods in binary cannot be overemphasized. The single greatest source of mistakes when working with subnets is trying to operate with them in dotted decimal without having understanding what exactly is happening in the binary level. Once more, dotted decimal is for comfort in reading and writing IPv4 addresses. Routers and hosts see the addresses as 32-bit binary strings; to successfully operate with these addresses, they must be seen the way the routers and hosts see them.

The previous paragraph may well seem a little overzealous in light of the examples given so far; the patterns of subnet and host addresses have been rather apparent without having having to determine the addresses and masks in binary. The next section makes use of the four design methods to derive a subnet design in which the dotted-decimal representations aren't so apparent.

Breaking the Octet Boundary

Inside the examples given so far, the subnet spaces have fallen on octet boundaries. This arrangement will not be often essentially the most practical or efficient decision. What if, for instance, you will need to subnet a Class B address across 500 data links, each and every using a maximum of 100 hosts? This requirement is effortlessly met, but only by applying nine bits within the subnet field: 29 2 = 510 obtainable subnets, leaving seven bits for the host field, and 27 2 = 126 obtainable hosts per subnet. No other bit combination will satisfy this requirement.

Notice, also, that there is certainly no approach to subnet a class C address on an octet boundarydoing so would use up all of the last byte, leaving no space for host bits. The subnet bits and host bits will need to share the last octet, as the following instance shows.

Figure 1-8 shows the network of Figure 1-7 but using a Class C address of 192.168.100.0 assigned.

Figure 1-8. The network from Figure 1-7 but using a Class C prefix assigned. Subnetting an complete octet won't operate right here; there would be no space left for host bits.[View complete size image]

image

You will discover 5 data links; for that reason, the address must be subnetted to present for no less than 5 subnet addresses. The illustration also indicates the quantity of hosts (such as router interfaces) that ought to be addressed on each and every subnet. The maximum host address requirement is 25 for the two Ethernets. For that reason, the complete subnetting requirements are no less than 5 subnets and no less than 25 host addresses per subnet.

Applying the 2n 2 formula, 3 subnet bits and 5 host bits will satisfy the requirements: 23 2 = 6 and 25 2 = 30. A Class C mask with 3 bits of subnetting is represented as 255.255.255.224 in dotted decimal.

Figure 1-9 shows the derivation of the subnet bits. The subnet mask derived in Step 2 is written in binary, plus the IP address is written beneath it. Vertical lines are drawn as markers for the subnet space, and inside this space all potential bit combinations are written by counting up from zero in binary.

Figure 1-9. The subnet bits are derived by marking the masked subnet bit space after which writing all potential bit combinations within the space by counting up from zero in binary.

image

In Figure 1-10, the unchanged network bits are filled in for the left of the subnet space plus the host bits, that are all zeros within the subnet addresses, are filled in for the correct of the subnet space. The outcomes are converted to dotted decimal, and these are the six subnet addresses (remembering that the initial and last addresses, which have 000 and 111 within the subnet space, cannot be utilised).

Figure 1-10. The subnet addresses are derived by filling within the network address for the left of the subnet space, setting all host bits to zero for the correct of the subnet space, and converting the outcomes to dotted decimal.

image

The last step is always to calculate the host addresses obtainable to each and every subnet. This step is performed by deciding on a subnet and, keeping the network and subnet bits unchanged, writing all bit combinations within the host space by counting up from zero in binary. Figure 1-11 shows this step for subnet 192.168.100.32.

Figure 1-11. The host addresses to get a subnet are derived by writing all potential bit combinations within the host space. These are the host bits for subnet 192.168.100.32.

image

Notice the patterns within the outcomes: The initial address, in which the host bits are all zero, could be the subnet address. The last address, in which the host bits are all a single, could be the broadcast address for subnet 192.168.100.32. The host addresses count up from the subnet address for the broadcast address, and when the sequence had been to continue, the next address would be the second subnet, 192.168.100.64.

The value of understanding subnetting in the binary level ought to now be clear. Presented with an address for instance 192.168.100.160, you can't be sure regardless of whether it is a host address, a subnet address, or a broadcast address. Even when the subnet mask is recognized, things aren't often readily apparent.

Readers are encouraged to calculate all host addresses for all of the remaining subnets within the instance and to observe the patterns that outcome within the addresses. Understanding these patterns will help in situations for instance the a single presented within the subsequent section.

Troubleshooting a Subnet Mask

The necessity frequently arises to "dissect" a given host address and mask, normally to identify the subnet to which it belongs. For instance, if an address is always to be configured on an interface, a great practice is always to very first verify that the address is valid for the subnet to which the subnet is connected.

Use the following methods to reverse-engineer an IP address:

Step 1.

Write the given subnet mask in binary.

Step 2.

Write the IPv4 host address in binary.

Step 3.

Being aware of the class of the host address, the subnet bits of the mask need to be apparent. Applying the mask bits as a guide, draw a line between the last network bit plus the very first subnet bit of the address. Draw another line between the last subnet bit plus the very first host bit.

Step four.

Write the network and subnet bits of the address, setting all host bits to zero. The outcome could be the address of the subnet to which the host address belongs.

Step 5.

Once more write the network and subnet bits of the address, this time setting all host bits to a single. The outcome could be the broadcast address of the subnet.

Step 6.

Being aware of that the subnet address could be the very first address within the sequence and that the broadcast address could be the last address within the sequence, you also know that all addresses between these two are valid host addresses.

Figure 1-12 shows these methods applied to 172.30.0.141/25.

Figure 1-12. Given an IPv4 address plus a subnet mask, comply with these methods to find the subnet, the broadcast, plus the host addresses.

image

The address can be a Class B, so it is recognized that the initial 16 bits are the network bits; for that reason, the last nine bits of the 25-bit mask mark the subnet space. The subnet address is located to be 172.30.0.128, plus the broadcast address is 172.30.0.255. Being aware of that the valid host addresses for the subnet are bounded by these two addresses, it is determined that the host addresses for subnet 172.30.0.128 are 172.30.0.129 through 172.30.0.254.

Numerous things about this instance have a tendency to bother folks who are new to subnetting. Some are bothered by the third octet of the address, that is all zeros. Some are bothered by the single subnet bit within the last octet. Some feel that the broadcast address looks suspiciously invalid. All of these uneasy feelings arise from reading the addresses in dotted decimal. When the addresses plus the mask are seen in binary, these suspicions are assuaged and every thing is seen to be legitimate; the mask sets a nine-bit subnet spaceall of the third octet, plus the very first bit of the fourth octet. The moral of the story is the fact that if every thing is recognized to be correct in binary,