SUBNETTING

A subnet allows the flow of network traffic between hosts to be segregated based on a network configuration. By organizing hosts into logical groups, subnetting can improve network security and performance.

Subnet Mask

Perhaps the most recognizable aspect of subnetting is the subnet mask. Like IP addresses, a subnet mask contains four bytes (32 bits) and is often written using the same "dotted-decimal" notation. For example, a very common subnet mask in its binary representation

11111111 11111111 11111111 00000000

is typically shown in the equivalent, more readable form

255.255.255.0

Applying a Subnet Mask

A subnet mask neither works like an IP address, nor does it exist independently from them. Instead, subnet masks accompany an IP address and the two values work together. Applying the subnet mask to an IP address splits the address into two parts, an "extended network address" and a host address.

For a subnet mask to be valid, its leftmost bits must be set to '1'. For example,

00000000 00000000 00000000 00000000

is an invalid subnet mask because the leftmost bit is set to '0'.

Conversely, the rightmost bits in a valid subnet mask must be set to '0', not '1'. Therefore,

11111111 11111111 11111111 11111111

is invalid.

All valid subnet masks contain two parts: the left side with all mask bits set to '1' (the extended network portion) and the right side with all bits set to '0' (the host portion).

Basic Subnetting Examples

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*** Example #1 ***
Original Network Address & Subnet Mask:	192.168.1.0 255.255.255.0
New Subnet Mask:	255.255.255.128
Bits Borrowed:	192.168.1.1|0000000
# of possible networks:	2
Each Sub-Network address increments by 128
1st Network	192.168.1.0
1st Usable	192.168.1.1
Last Usable	192.168.1.126
Broadcast	192.168.1.127
2nd Network	192.168.1.128
1st Usable	192.168.1.129
Last Usable	192.168.1.254
Broadcast	192.168.1.255
*** Example #2 *** Original Network Address & Subnet Mask: 192.168.6.0 255.255.255.0 New Subnet Mask: 255.255.255.192 Bits Borrowed: 192.168.6.11|000000 # of possible networks: 4 Each Sub-Network address increments by 64 1st Network 192.168.6.0 1st Usable 192.168.6.1 Last Usable 192.168.6.62 Broadcast 192.168.6.63 2nd Network 192.168.6.64 1st Usable 192.168.6.65 Last Usable 192.168.6.126 Broadcast 192.168.6.127 3nd Network 192.168.6.128 1st Usable 192.168.6.129 Last Usable 192.168.6.190 Broadcast 192.168.6.191 4nd Network 192.168.6.192 1st Usable 192.168.6.193 Last Usable 192.168.6.254 Broadcast 192.168.6.255