IP addresses, when it was created a few decades ago, used the concept of classes. This architecture is called classful addressing. In the mid-1990s, a new architecture called classless addressing, was introduced which eventually superseded the original architecture. However, part of the Internet still uses classful addressing, though the migration is going very fast. In this article, I am going to analyze classful IPv4 Networks.

Originally, a 32-bit IPv4 address was logically subdivided into two: the network number field, which is the most-significant 8 bits of an address since it specified the particular network a host was attached to; and the local address, also called rest field (the rest of the address), which uniquely identifies a host connected to that network. This format was sufficient at a time when only a few large networks existed, such as the ARPANET which was assigned the network number 10, and before the wide proliferation of local area networks (LANs). As a consequence of this architecture, the address space supported only a low number (254) of independent networks, and it became clear very early on that this would not be enough.

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The first class, designated as Class A, contained all addresses in which the most significant bit is zero. The network number for this class is given by the next 7 bits, therefore accommodating 128 networks in total, including the zero networks, and including the existing IP networks already allocated.

Class B network was a network in which all addresses had the two most-significant bits set to 1 and 0. For these networks, the network address was given by the next 14 bits of the address, thus leaving 16 bits for numbering hosts in a network, for a total of 65,536 addresses per network.

Class C was defined with the 3 high-order bits set to 1, 1, and 0, designating the next 21 bits to number the networks, leaving each network with 256 local addresses.

The leading bit sequence 111 designated an “escape to extended addressing mode“, which was later subdivided into Class D (1110) for multicast addressing, while leaving as reserved for future use the 1111 block designated as Class E.


The number of addresses usable for addressing specific hosts in each network is always 2N-2, where N is the number of rest field bits. The subtraction of 2 adjusts for the use of the all-bits-zero host portion for the network address and the all-bits-one host portion as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the number of hosts is 254.

A Classful network is a network-addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing in 1993. The method divides the address space for Internet Protocol Version 4 (IPv4) into five address classes. Each class, coded in the first four bits of the address, defines either a different network size, i.e. number of hosts for unicast addresses (classes A, B, C), or a multicast network (class D). The fifth class (E) address range is reserved for future or experimental purposes.

Since its discontinuation, remnants of class full network concepts remain in practice only in limited scope such as in the default configuration parameters of some network software and hardware components (e.g. default subnet mask), but the terms are often still used erroneously by people working in IT.

Today, IP addresses are associated with a subnet mask. This was not required in a classful network because the mask was implicitly derived from the IP address itself. Any network device would only need to inspect the first few bits of the IP address to determine the class of the address.

Bit wise representation

  • n indicates a binary slot used for network ID.
  • H indicates a binary slot used for host ID.
  • X indicates a binary slot (without specified purpose)

Class A

0. 0. 0. 0 = 00000000.00000000.00000000.00000000
127.255.255.255 = 01111111.11111111.11111111.11111111
0nnnnnnn.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class B

128. 0. 0. 0 = 10000000.00000000.00000000.00000000
191.255.255.255 = 10111111.11111111.11111111.11111111
10nnnnnn.nnnnnnnn.HHHHHHHH.HHHHHHHH

Class C

192. 0. 0. 0 = 11000000.00000000.00000000.00000000
223.255.255.255 = 11011111.11111111.11111111.11111111
110nnnnn.nnnnnnnn.nnnnnnnn.HHHHHHHH

Class D

224. 0. 0. 0 = 11100000.00000000.00000000.00000000
239.255.255.255 = 11101111.11111111.11111111.11111111
1110XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

Class E

240. 0. 0. 0 = 11110000.00000000.00000000.00000000
255.255.255.255 = 11111111.11111111.11111111.11111111
1111XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

For example, 162.22.39.1 is a Class B address because the first octet, 162, lies in the 128-191 range. Likewise, 10.11.26.1 is a Class A addresses (because the first octet is 10) and 204.10.209.1 is a Class C (because the first octet is 204). If this seems confusing, convert these addresses into binary and verify for yourselves that the initial bits correspond to the pattern shown in the diagram illustrated above.

The network engineer would request a Class A, B, or C network upon installing a new Internet connection, depending on the expected size of the installed network. For example, the U.S. Department of Defense, a very large network, was assigned a Class A; the Queens University of Belfast in the United Kingdom, a typical mid-sized network, was assigned a Class B network; while a small consulting Institution I once worked for was assigned a Class C network.

The Internet Assigned Numbers Authority (IANA) oversaw all classful network assignments. Only the network bits were assigned by IANA. For example, a request for a Class C network might have been met by assigning 192.12.26.0. As a Class C, the first three bytes were fixed by IANA, and the last byte was assigned by the local network administrator. No attempt was made to assign the addresses in a hierarchical fashion. The first Class B assigned was 128.1.0.0; the next was 128.2.0.0, and so on.

Routers process packets according to their classful network. For example, consider a packet addressed to 146.29.55.2. First, the address is determined to be a Class B (its two high bits are 10), then it’s split to determine its membership in the 146.29.0.0 classful network. The routing table would have an entry for each classful network, in this case 146.29.0.0, which would determine how the packet should be delivered.

Problems with class subnetting

The original classful address scheme had a number of problems:

Scenario I: Very few network addresses for large networks

  • Class A and Class B addresses are gone, so only Class C will be suitable for this scenario.

Scenario II: Two-layer hierarchy is not appropriate for large networks with Class A and Class B
addresses.
Solution: Subnet the networks.

Scenario III: Assume a company requires 2,000 addresses, what would their network engineer do?

  • Class A and B addresses are overkill.
  • Class C address is insufficient (requires 8 Class C addresses).

Solution: Use Classless Inter-domain Routing (CIDR).

Scenario IV – Exploding Routing Tables: Routing on the backbone Internet needs to have an entry for each network address. In 1993, the size of the routing tables started to outgrow the capacity of routers.

  • Solution: Classless Inter-domain Routing (CIDR)

Scenario V: The Internet is going to outgrow the 32-bit addresses.

  • Solution: IP Version 6

So, the overall solution would be achieved by using classless inter-domain routing. The table below describes how many hosts can be saved with classless networks.


For example if we only need two hosts, we can apply /30 CIDR, then 4 hosts are available inclusive of one network-id and a broadcast-id. We get a total of two valid hosts by using /30 CIDR and there won’t be any host wastage.

Tips for CCNA exams:

I hope that after reading this article, you will be able to understand Classful Addressing and the requirements of Classless Addressing. For the CCNA examination, I recommend that you read all my previous articles on IPv4 addressing and subnetting to master these topics which are very much important for both the exams and day to day job environments. Most importantly, after reading this article you need to practice and analyze networks on your own to excel in this technology.

Keep reading my articles to enhance your technical skills. Your feedback and comments are always welcome.

Reference:

  1. Guide to Cisco Certified Network Associate certification by Todd Lamlee, Sybex press.
  2. Guide to Cisco Certified Network Associate by Richard Deal.
  3. Cisco Certified Network Professional-Route by Wendell Odom, Ciscopress.com
  4. CCNP- Route Quick reference by Denis Donohue, Ciscopress.com
  5. Cisco Certified Internetwork Expert by Wendell Odom and others, Ciscopress.com
  6. Cisco Certified Internetwork Expert Quick reference by Brad Ellis, Ciscopress.com