Subnetwork

The word subnetwork (usually shortened to subnet) has two related meanings. In the older and more general meaning, it meant one physical network of an internetwork. In the Internet Protocol (IP), a subnetwork is a division of a classful network. The rest of this article is about the second meaning.

Subnetting an IP network allows a single large network to be broken down into what appear (logically) to be several smaller ones. It was originally introduced before the introduction of classful network numbers in IPv4, to allow a single site to have a number of local area networks. Even after the introduction of classful network numbers, subnetting continued to be useful, as it reduced the number of entries in the Internet-wide routing table (by hiding information about all the individual subnets inside a site). As a side benefit, it also resulted in reduced network overhead, by dividing the parts which receive IP broadcasts.

Network masks

A network mask, also known as a subnet mask, netmask or address mask, is a bitmask used to tell how many bits in an octet(s) identify the subnetwork, and how many bits provide room for host addresses.

Subnet masks are usually represented in the same representation used for addresses themselves; in IPv4, dotted decimal notation, four numbers from zero to 255 separated by periods, e.g. 255.128.0.0. Since the mask consists of only a series of all ones followed by all zeroes, only those numbers representing such sequences are allowed: 0, 128, 192, 224, 240, 248, 252, 254, and 255.

Less commonly, it can be represented as an eight-digit hexadecimal number (e.g. FF.80.00.00 = 255.128.0.0).

A shorter form, which is known as Classless Inter-Domain Routing (CIDR) notation, gives the network number followed by a slash ("/") and the number of 'one' bits in the binary notation of the netmask (i.e. the number of relevant bits in the network number). For example, 192.0.2.96/28 indicates an IP address where the first 28 bits are used as the network address (same as 255.255.255.240).

Subnetworking concept

IPv4 addresses are broken down into three parts: the network part, the subnet part (now often considered part of the network part, although originally it was part of the rest part), and the host part. There are three classes of IP address which determine how much is which.

Class	Leading bits	Start	End	Default Subnet Mask in dotted decimal	CIDR notation
A	0	1.0.0.0	126.0.0.0	255.0.0.0	/8
B	10	128.0.0.0	191.255.0.0	255.255.0.0	/16
C	110	192.0.0.0	223.255.255.0	255.255.255.0	/24
D	1110	224.0.0.0	239.255.255.0
E	1111	240.0.0.0	255.255.255.0

The 127.0.0.1 Network ID is left out because it is designated for loopback and cannot be assigned to a network

Class D multicasting

Class E reserved

Subnetting is the process of allocating bits from the host portion as a network portion. For example, giving the class A network 10.0.0.0 a subnet mask of 255.255.0.0 would break it down into 256 sub-networks (10.0.0.0 to 10.255.0.0), and indicates that the first octet of the IP address shows the network address, the second one shows the subnet number and the last two show the host part. A bitwise AND operation of the host address with the subnet mask extracts the complete subnetwork address (see example below).

Subnet masks are not limited to whole octets, either. For example 255.254.0.0 (or /15) is also a valid mask. Applied to a class A address this would create 128 subnetworks in intervals of two (1.2.0.1 - 1.3.255.254, 1.4.0.1 - 1.5.255.254, etc).

Example

Having the IP address 12.11.10.9, with a subnet mask of 255.254.0.0 (the same as 12.11.10.9/15) says:

Host address 12.11.10.9 (decimal) => 00001100 00001011 00001010 00001001 (binary).

The most significant bit is zero, therefore we have a class 'A' network with the network address 12.0.0.0 (see classful network).

Subnet mask 255.254.0.0 (decimal) => 11111111 11111110 00000000 00000000 (binary).

The subnet mask extends the network address by 7 more bits (254). Thus we have a subnet number of 10 and an extended network address of 12.10.0.0 (decimal) = 00001100 00001010 00000000 00000000 (binary) (bitwise AND of 12.11.10.9 with 255.254.0.0).

The remaining host part is 0.1.10.9 (decimal) => 00000000 00000001 00001010 00001001 (binary).

Determining the number of hosts and subnets on a particular network is quite easy, if you know the subnet mask. Say you have the network address 154.4.32.0 with a subnet mask of 255.255.224.0. This network address can also be written as 154.4.32.0/19

Network address 154.4.32.0 (decimal) => 10011010.00000100.00100000.00000000 (binary)

Subnet mask 255.255.224.0 (decimal) => 11111111.11111111.11100000.00000000 (binary)

The subnet mask has 19 bits for the network portion of the address, and 13 bits for the host part.

$2^{13} = 8192$
possible subnets available according to RFC 1812, otherwise using the old RFC 950 standard the number of usable subnets is 6. This is due to RFC 950 (section 2.1, page 5) not supporting subnets with either all 1s or all 0s.

$2^{13} - 2 = 8190$
possible hosts available to each subnetwork.

Host and Subnet Counts

It is possible to determine the amount of subnets required in each subnet mask along with the amount of hosts.

For example, we will use the class B subnet 255.255.0.0

This is written as 11111111.11111111.00000000.00000000 in binary form.

By adding extra mask bits to our subnet, we can determine the amount of subnets.

11111111.11111111.11000000.00000000 => 255.255.192.0 (/18)

We can now work out the subnet count by getting 2 to the power of how many mask bits we added, in this case we have added 2: $2^{2} = 4$ which gives us the subnet number. To get the effective subnet count, we subtract 2 from this value.

Using the Least significant bit (lsb) of the subnet, we can find out how many hosts we have. The lsb of 11111111.11111111.11000000.00000000 is 1000000.00000000 => 16384. To get the effect host count, we subtract 2 from this value. This is because of the need for a network and broadcast address.

Private subnets

The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private networks:

Network address range	CIDR notation
10.0.0.0 - 10.255.255.255	/8
172.16.0.0 - 172.31.255.255	/12
192.168.0.0 - 192.168.255.255	/16

Nonstandard subnets

Subnet zero

Technically illegal, but still usable, is the extreme first subnetwork; for example, subnet 1.0.0.0 with a subnet mask address of 255.255.0.0. The problem with this subnet is that the unicast address for the subnet is the same as the unicast address for the entire class A network.

Interleaved subnets

External links

Routing | Network addressing | Internet architecture

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