TCP/IP Subnetting Example

IP SUBNETTING – MANTRA ********

1) What Sub-net Mask Should I use ?

How many subnets are required ? = 27

Convert this number to low order Binary = 00011011

How many bits used to represent number ? = 5

Convert number of bits used to high order binary and convert to decimal = 248

2) How many subnets do I actually get using this mask ?

128 64 32 16 8 4 2 1

1 1 1 1 1 0 0 0
How Many bits were used for the subnet mask = 5
25 - 2 = no of subnets = 30

3) What are my subnet IDs

128 64 32 16 8 4 2 1

1 1 1 1 1 0 0 0 = 8


Starting at this number jump in increments of the same value (eg 8,16,24,32 etc..)
Until 1 below the subnet mask value ( i.e upto 240) There should be as many Ids as your calculated subnets figure.


4) How many HOSTS do I get per Subnet?

How many bits are remaining for Host ID? = 3 from third octet and 8 from fourth octet ie = 11 bits

11111111 11111111 11111000 00000000
255 255 248 0


2 11 -2 no of hosts per subnet = 2,046




5) What other HOSTS are on my Subnet ?
What value did you increment by {see 2) } = 8s
Which range does the subnet portion of your IP address fit into ?
Are the other hosts in the same range as you ?

Example
If your IP address is 131.107.26.10 with a subnet mask of 255.255.248.0 you fit in the range
Between 131.107.24.1 -> 131.107.31.254


Are these hosts in the same range as you
131.107.24.12 = yes
131.107.35.1 = no
131.107.56.20 = no
131.107.31.254 = yes