How the extended network prefixes work

Well, the subnet mask is in charge of this. The subnet mask looks for the network prefix to ensure that a synchronized job is being processed. For example, if you need to specify a subnet mask in a class network, any subnet that you will specify will define your network

Fig 8.7

Even though the subnet mask is in charge of the lookup, the router protocol refers to the extended network prefix length rather than the actual subnet mask. Therefore the prefix length is contiguous 1bit in the subnet mask and because there is no standard which defines it, the routing protocol takes place to carry out the subnet mask. If the protocol fails to identify the subnet mask, the network just wouldn't exist, and all current process would be terminated.

Network Engineering, Designing Subnets

Example of Sub-netting:
If a company has been assigned a network number 192.168.1.0/24 and it needs to define six subnets; it is required to support at least 28 hosts for one of their departments.

16 (2⁴), 8 (2³), 4 (2²), 2 (2¹)

First we need to determine the number of bits required for the six subnets.

• Observe the power notation above, it is impossible to define six, therefore 2³= 8 (3-bits are required). We have 2 unused subnets.
• We are subnetting a /24 bit network prefix, our calculation result, was 3 bits required to define the 8 subnets; then 24+3 = 27 this makes our network an extended 27-bit prefix.

Fig 8.8

Note. An IP address is a set of 4 octets of 8 bits.

• This leaves us 5 bits for our hosts in each subnet. If we do the calculation for the five bits 2⁵ gives us 32.

You may be wondering how we got that 224 subnet mask. The subnet mask is based on the 3-bits result of our 8 sub networks. Check the following table.

Subnet Expansion
Bit Pattern	Masked Bits	Provided Subnets	Subnet Mask
11000000 2	2	2	192
11100000 3	3	6	224
11110000 4	4	14	240
11111000 5	5	30	248
11111100 6	6	62	252
11111110 7	7	126	254
11111111 8	8	254	255

Table 8.2

Yes, I know what you are saying, “The table doesn't tell me anything”. Look, this is how it goes:

Bit value	128	64	32	16	8	4	2	1
Subnet mask	-	192	224	240	248	252	254	255
#of SubNework	-	2	6	14	30	62	126	254

Table 8.3

Here we go again, 3 bits was required for our 8 subnets, now look here:

Table 8.4

128 + 64 = 192	----	Number of Sub-networks
192 + 32 = 224		If 192 Subnet mask is used it
224 + 16 = 240		will give 2 subnets.
240 + 8 = 248
248 + 4 = 252		If 224 Subnet mask is used it
252 + 2 = 254		will give 6 subnets.
254 + 1 = 255