Subnet and host counts

The number of subnetworks available, and the number of possible hosts in a network may be readily calculated. In the example (above) two bits were borrowed to create subnetworks, thus creating 4 (22) possible subnets.

Network Network (binary) Broadcast address 11000000.10101000.00000101.00000000 11000000.10101000.00000101.01000000 11000000.10101000.00000101.10000000 11000000.10101000.00000101.11000000

The RFC 950 specification recommended reserving the subnet values consisting of all zeros (see above) and all ones (broadcast), reducing the number of available subnets by two. However, due to the inefficiencies introduced by this convention it was abandoned for use on the public Internet, and is only relevant when dealing with legacy equipment that does not implement CIDR. The only reason not to use the all-zeroes subnet is that it is ambiguous when the prefix length is not available. RFC 950 itself did not make the use of the zero subnet illegal; it was however considered best practice by engineers.

CIDR-compliant routing protocols transmit both length and suffix. RFC 1878 provides a subnetting table with examples.

The remaining bits after the subnet bits are used for addressing hosts within the subnet. In the above example the subnet mask consists of 26 bits, leaving 6 bits for the host identifier. This allows for 62 host combinations (26-2).

The all-zeros value and all-ones values are reserved for the network address and broadcast address respectively. In systems that can handle CIDR a count of two is therefore subtracted from the host availability, rather than the subnet availability, making all 2n subnets available and removing a need to subtract two subnets.

For example, under CIDR /28 all 16 subnets are usable. Each broadcast, i.e. .15 .31 – .255 comes off the client count, not the network, thus making the last subnet also usable.

In general the number of available hosts on a subnet is 2h-2, where h is the number of bits used for the host portion of the address. The number of available subnets is 2n, where n is the number of bits used for the network portion of the address. This is the RFC 1878 standard used by the IETF, the IEEE and COMPTIA.

RFC 3021 specifies an exception to this rule for 31-bit subnet masks, which means the host identifier is only one bit long for two permissible addresses. In such networks, usually point-to-point links, only two hosts (the end points) may be connected and a specification of network and broadcast addresses is not necessary.

A /24 network may be divided into the following subnets by increasing the subnet mask successively by one bit. This affects the total number of hosts that can be addressed in the /24 network (last column).

Prefix size Network mask Available
Usable hosts
per subnet
usable hosts
/24 1 254 254
/25 2 126 252
/26 4 62 248
/27 8 30 240
/28 16 14 224
/29 32 6 192
/30 64 2 128
/31 128 2 * 256