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IPv4

Understanding IPv4 Addressing

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Introduction to IPv4 Addresses

IPv4 addresses are 32-bit numbers displayed in dotted decimal notation. The IPv4 address consists of two parts, the Network and the Host.

Hosts within a network share the same network address. The host has an address that identifies it and is unique to the network it belongs to. This address can be globally or locally unique depending on the scope of the network and the type of device. Any host that are visible outside of the network, such as web servers hosting a web site, must have a globally unique IP address. Hosts within the confines of the local network must have a locally unique address.

IP addresses are assigned by a central numbering authority known as the Internet Assigned Numbers Authority (IANA), is assured that addresses that are globally unique.

There are three types of addresses that are used within the IPv4 Network range.

  • Network address - The address that refers to the network.
  • Broadcast address - The address used to send data to all hosts.
  • Host address - The address that is assigned to the host.

Classless and Classful Addressing Differences

Classless Addressing

Within a classless addressing system, address blocks of appropriate size to the number of hosts are assigned to companies or organizations without regard to the unicast class. Classless addressing is associated with such things as a network of fixed sized.

Classful Addressing

Within a classful addressing system, the use of addressing space consists of class A, class B or class C addressing blocks. There are classes D and E, but class D is used for multi-cast and class E is for experimentation and not used in regular IP addressing schemes.

Classful Addressing

Since classful addressing is most commonly used with IPv4, we will focus on this type of addressing for discussion.

The Classful Address Blocks

As previously stated, the classful addressing blocks are class A, B, C, D and E. Also previously stated, class blocks D and E are reserved for multi-cast and experimental purposes respectively. Here are the summaries of class A, B and C address blocks.

Class 1st Octet Range Prefix & Mask # of Networks # of Hosts
A 1 - 127 /8 255.0.0.0 126 (27) 16,777,214 (224-2)
B 128 - 191 /16 255.255.0.0 16,384 (214) 65,534 (216-2)
C 192 - 223 /24 255.255.255.0 2,097,159 (221) 254 (28-2)
D 224 - 239
E 240 - 254

For clarification, see below the representation of the Network and the Host portion to the classes A,B and C blocks. Networks represented as 'N' and Hosts represented as 'H'.

Class 1stOctet 2ndOctet 3rdOctet 4thOctet
A N H H H
B N N H H
C N N N H

Private and Public Addresses

Most IP addresses are public in nature and designed for use in networks that are accessible on the Internet. There are blocks of addresses used in networks that require limited or no Internet access and these are known as private addresses. The private address blocks are:

  • 10.0.0.0 /8 (10.0.0.0 - 10.255.255.255.255) -Class A
  • 172.16.0.0 /12 (172.16.0.0 - 172.31.255.255) -Class B
  • 192.168.0.0 /16 (192.168.0.0 - 192.168.255.255 -Class C

Since these addresses are set aside for private networks they do not have to be unique in the outside world. They do, or may need to be able to access outside the local network. Additionally, many hosts in different networks can uses these same private addresses. Packets using these addresses as the source or destination should not appear on the public Internet. This is where Network Address Translation (NAT) comes into play. NAT can be implemented on an edge device on the private network. NAT changes the private address in the IPv4 packet header to a public IP address. By doing so, the internal hosts can communicate with the outside world.

Special Unicast IPv4 Addresses

Special addresses can be assigned to hosts but under the restrictions on how the hosts can interact within the network. These special addresses include:

  • Default Route
  • Loopback Address
  • Link-local Address
  • Test-net Address

Default Route - Is the default "catch all" route to route packets when a more specific route is not available. The IPv4 default route is 0.0.0.0.

Loopback - is another reserved address block ranging from 127.0.0.0 to 127.255.255.255 (127.0.0.0 /8). The loobback is a special address that hosts use to direct traffic to themselves.

Though only 127.0.0.1 is used out of this block, the whole block is reserved. Any of these addresses will loopback and you should never see them on any network.

Link-Local - This address is the 169.254.0.0 to 196.254.255.255 (169.254.0.0 /16) block. These addresses can be automatically assigned to the local host by the operating system in environments where no IP configuration is available.

Test-Net Addresses - This address block is 192.0.2.0 to 192.0.2.255 (192.0.2.0 /24). These addresses can be used in documentation and network examples.