Internet Protocol Version 4 (IPv4)

networks

Anatomy

An IPv4 address is divided into two major portions using a subnet mask:

1. Network Portion
2. Host Portion

By ANDing an IPv4 address with its subnet mask, the network portion and the host portion can be distinguished.

Network Portion

Network Portion

The sequence of IPv4 address bits where the corresponding subnet mask bit is $1$.

Host Portion

Host Portion

The sequence of IPv4 address bits where the corresponding subnet mask bit is $0$.

Subnetting

Subnetting is the process of dividing an IPv4 base address into smaller segments, called “subnets”.

Benefits: By subnetting an IPv4 space, the broadcast domain is divided into smaller segments, which results in less likely network congestions.
Also, security rules, such as access lists, can be configured for more fine-grained security measurements.

Cumulated Network Bits

The cumulated network bits is the number of bits at the higher end of the octet in which the prefix is located. It is defined as:

$$\text{Cumulative Network Bits}=\{\begin{array}{ll}32& \text{if Prefix}\ge 24\\ 24& \text{if Prefix}\ge 16\\ 16& \text{if Prefix}\ge 8\\ 8& \text{else}\end{array}$$

Block Size

The block size determines the address space size of a subnet.

$$\text{Block Size}=2^{\text{Cumulative Network Bits}-\text{Prefix}}$$

Network Address

The network address is the first address in a subnet’s address space. When all host bits are zeros, an IPv4 address is considered a network address.

Broadcast Address

The broadcast address is the last address in a subnet’s address space. When all host bits are ones, an IPv4 address is considered a broadcast address.

Available Hosts

The available hosts are determine by the block size:

$$\text{Available Hosts}=\text{Block Size}-2$$

All hosts in the address space are available, except the network and broadcast address. Therefore, 2 is subtracted from the block size.

Practice

• IPv4 Subnetting Practice