1 IPv4 Addresses, Subnet Masks, and CIDR

IPv4 address structure

An IPv4 address is $32$ bits long and is divided into four groups of $8$ bits, called octets. Each octet is written as a decimal number from $0$ to $255$, and periods separate the octets. For example, 192.168.1.70 contains four octets.

Subnet masks and CIDR notation

A subnet mask separates the bits that belong to the network prefix from the bits available for addresses within that prefix. In a standard CIDR mask, the network bits are contiguous: they are 1 bits from the left, followed only by host bits represented by 0 bits.

CIDR notation writes an address followed by a slash and a number from $0$ to $32$. That number is the prefix length. For example, 192.168.1.70/26 has $26$ prefix bits and $6$ remaining host bits. CIDR makes the prefix length explicit instead of inferring it from an address’s historical class.

Convert a prefix length to a mask

To convert a prefix length to a dotted-decimal mask, each complete group of $8$ prefix bits becomes an octet of 255. If the prefix ends partway through an octet, use the matching partial-octet value below; all following mask octets are 0.

• $0$ set bits: 00000000 = 0

• $1$ set bit: 10000000 = 128

• $2$ set bits: 11000000 = 192

• $3$ set bits: 11100000 = 224

• $4$ set bits: 11110000 = 240

• $5$ set bits: 11111000 = 248

• $6$ set bits: 11111100 = 252

• $7$ set bits: 11111110 = 254

• $8$ set bits: 11111111 = 255

For example, /26 fills the first three octets with $24$ prefix bits, giving 255.255.255. The remaining $2$ prefix bits make the next octet 11000000, or 192, so /26 is 255.255.255.192.

Convert a mask to a prefix length

To convert a dotted-decimal mask to a prefix length, count the 1 bits in each octet and add the counts. For example, 255.255.255.192 contains $8+8+8+2=26$ one-bits, so its prefix length is /26.

A valid CIDR subnet mask must have all its 1 bits together on the left, followed only by 0 bits. 255.255.255.160 is not a valid contiguous CIDR mask: its final octet is 10100000, which has a 1 after a 0.

Common masks and conversion summary

The following common CIDR prefixes correspond to these dotted-decimal masks:

• /8: 255.0.0.0

• /16: 255.255.0.0

• /24: 255.255.255.0

• /25: 255.255.255.128

• /26: 255.255.255.192

• /27: 255.255.255.224

• /28: 255.255.255.240

• /30: 255.255.255.252

In short, an IPv4 address has four octets, totaling $32$ bits. A subnet mask distinguishes prefix bits from remaining bits, while CIDR expresses the number of prefix bits as /n. To convert /n to a mask, use 255 for each complete octet, then the partial-octet value if needed. To convert a mask to CIDR, count its contiguous leading 1 bits.