Method of subnetting

Key terms to remembers

·    A subnet is a smaller portion of large network treated as its own separate network. To create subnet we borrow bits from host portion and assign them as network bits. This mean more networks, fewer hosts.

·    If the network bits on two addresses do not match, then the two packets are intended for two separate networks.

·    On a 32 bits IP address at least eight bits must belong to the network portion and at least 2 bits must belong to the host portion.

·    Each IP address has a predefined IP class and that cannot be changed.

·    Each class has a predefined default subnet mask that tell us the octets, which are already part of the network portion, as well as how many bits we have available to work with.

·    Whatever network class is it, we cannot change those bits that are already assigned.

·    We cannot assign the network ID and the broadcast address to a host.

·    Regardless how many bits are left in the host field, network ID and the broadcast address must be reserved.

·    Subnet bits start at the left and go to the right, without skipping bits.

CIDR [ Classless Inter Domain Routing]

CIDR is a slash notation of subnet mask. CIDR tells us number of on bits in a network address.

·    Class A has default subnet mask 255.0.0.0. that means first octet of the subnet mask has all on bits. In slash notation it would be written as /8, means address has 8 bits on.

·    Class B has default subnet mask 255.255.0.0. that means first two octets of the subnet mask have all on bits. In slash notation it would be written as /16, means address has 16 bits on.

·    Class C has default subnet mask 255.255.255.0. that means first three octets of the subnet mask have all on bits. In slash notation it would be written as /24, means address has 24 bits on.

Method of subnetting

In subnetting we find the answer of following questions.

·      What is subnet mask for given address?

·      How many subnets does given subnet mask provide ?

·      What is block size for given subnet mask?

·      What are the valid subnets?

·      What are the total hosts?

·      How many valid hosts are available per subnet?

·      What is broadcast address of each subnet?

·      What is network address of each subnet?

To answer above questions we use following method of subnetting.

What is subnet mask for given address?

Subnetting take places when we extend the default subnet mask. We cannot perform subnetting with default subnet mask and every classes have default subnet mask. To figure out subnetted subnet mask ,we first need to write down the default subnet mask. Now find the host bits borrowed to create subnets and convert them in decimal. For example find the subnet mask of address 188.25.45.48/20 ? This address belong to class B and class B has default subnet mask 255.255.0.0[ /16 in CIDR ]. We borrowed 4 bits from hosts portion. As you know subnetting move from left to right and it cannot skip any network bit. So this subnet mask in binary would be 11111111. 11111111.11110000.00000000. First two octet have default value so its decimal value would be 255.255. We will convert third octet in decimal value. To convert a binary number in decimal we add its decimal equivalent value. In our example it would be 128+64+32+16+0+0+0+0 = 240. Our fourth octet has all bits off so its decimal value would be 0+0+0+0+0+0+0+0 =0. Our answer subnet mask would be 255.255.240.0

How many subnets does given subnet mask provide ?

To calculate the number of subnets provided by given subnet mask we use 2^N , where N = number of bits borrowed from host bits to create subnets. For example in 192.168.1.0/27, N is 3. By looking at address we can determined that this address is belong to class C and class C has default subnet mask 255.255.255.0 [/24 in CIDR]. In given address we borrowed 27 - 24 = 3 host bits to create subnets. Now 2³ = 8, so our answer is 8.

What is block size for subnet mask?

Block size or increment number is used to calculate the valid subnets. Once you figure out the block size, calculation of valid subnets become piece of cake. To figure out the block size, use this formula 256 - Subnet mask = block size. For example block size for subnet mask 255.255.255.240 is 256 - 240 = 16.

What are the valid subnets?

Calculating valid subnet is two steps process. First calculate total subnet by using formula 2^N. In second step find the block size and count from zero in block until you reach the subnet mask value. For example calculate the valid subnets for 192.168.1.0/26.

Borrowed host bits are 2 [26-24].
Total subnets are 2² = 4.
Subnet mask would be 255.255.255.192.
Block size would be 256-192 = 64.
Start counting from zero at blocks of 64, so our valid subnets would be 0,64,128,192.

What are the total hosts?

Total hosts are the hosts available per subnet. To calculate total hosts use formula 2^H = Total hosts. H is the number of host bits. For example in address 192.168.1.0/26 we have 32 - 26 [Total bits in IP address - Bits consumed by network address] = 6. Total hosts per subnet would be 2⁶ = 64.

How many valid hosts are available per subnet?

Valid hosts are the number of hosts those can be assigned to devices. As we know, we need to reduce two address per subnet, one for network ID and another for broadcast ID. So our formula, to calculate valid hosts would be Total hosts - 2 = Valid hosts. In above example we have 64 hosts per subnet, so valid hosts in each subnet would be 64 - 2 = 62.

What is broadcast address of each subnet?

Broadcast address is the last address of subnet. This address is reserve for network broadcast, and cannot be assigned to any host. In above example

0 Subnet has broadcast address 63
64 Subnet has broadcast address 127
128 Subnet has broadcast address 191
192 Subnet has broadcast address 255

What is the network address of each subnet?

Network address is the first address of subnet. This address is used to locate the network, and cannot be assigned to any host. In above example address 0,64,128,192 are the network address.

·      Network address is always the first IP address of subnet.

·      Broadcast address is always the last IP address of subnet ( IP address before the next subnet).

·      Valid hosts are the IP addresses between network address and broadcast address.

At this point you have powered with all essential tools for subnetting. In last section of this article we will practically implement what we have learn so far. Due to length of this article I will include examples only from class C.

Class C Subnetting

Default subnet mask of class C is 255.255.255.0. CIDR notation of class C is /24, which means 24 bits from IP address are already consumed by network portion and we have 8 host bits to work with. We cannot skip network bit, when we turned them on. Subnetting moves from left to right. So Class C subnet masks can only be the following:

CIDR	Decimal	Binary
/25	128	10000000
/26	192	11000000
/27	224	11100000
/28	240	11110000
/29	248	11111000
/30	252	11111100

As we have already discussed earlier in this article that we have to have at least 2 host bits for assigning IP addresses to hosts, that means we can't use /31 and /32 for subnetting.

/25

CIDR /25 has subnet mask 255.255.255.128 and 128 is 10000000 in binary. We used one host bit in network address.

N = 1 [Number of host bit used in network]
H = 7 [Remaining host bits]
Total subnets ( 2^N ) :- 2¹ = 2
Block size (256 - subnet mask) :- 256 - 128 = 128
Valid subnets ( Count blocks from 0) :- 0,128
Total hosts (2^H) :- 2⁷ = 128
Valid hosts per subnet ( Total host - 2 ) :- 128 - 2 = 126

Subnets	Subnet 1	Subnet 2
Network ID	0	128
First host	1	129
Last host	126	254
Broadcast ID	127	255