Using default Class A, B and C subnets (Classful IP Addressing) is inefficient
It wastes unused IP Addresses (Private IP Addresses)
Subnetting allows to create multiple logical networks that exist within a single Class A, B or C network (Classless Subnet)
It also allows for more efficient routing via router summarization i.e. Routing tables do not have to store data for an large network so routing tables won’t become large
Fixed Length Subnetting
We borrow host bits to create smaller networks from a Class A, B or C network
We end up with more networks but the amount of IP addresses in each network reduces
Observation
255.255.255.X: Every bit we borrow results in the subnet getting divided in half
255.255.255.0: 1 Network
255.255.255.128: 2 Network
255.255.255.192: 4 Network
| x | Network | Addresses | ||||
|---|---|---|---|---|---|---|
| /1 | /9 | /17 | /25 | 128 | 2 | 128 |
| /2 | /10 | /18 | /26 | 192 | 4 | 64 |
| /3 | /11 | /19 | /27 | 224 | 8 | 32 |
| /4 | /12 | /20 | /28 | 240 | 16 | 16 |
| /5 | /13 | /21 | /29 | 248 | 32 | 8 |
| /6 | /14 | /22 | /30 | 252 | 64 | 4 |
| /7 | /15 | /23 | /31 | 254 | 128 | 2 |
| /8 | /16 | /24 | /32 | 255 | 256 | 1 |
| Addresses | Boundary Values |
|---|---|
| 128 | 0, 128 |
| 64 | 0, 64, 128, 192 |
| 32 | 0, 32, 64, 96, 128, 160, 192, 224 |
| 16 | 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240 |
Step 1: Using Chart convert IP Address and Subnet Mask to Decimal Notation
/26: 255.255.255.192
/24: 255.255.255.0
/20: 255.255.240.0
Step 2: Calculate Network Address
If subnet mask is 255 bring down the address
If its 0, use 0
For any other value based on value of x refer charts
Step 3: Calculate Broadcast Address
If subnet mask is 255 bring down the address
If its 0, use 255
For any other value based on value of x refer charts
Step 4: Calculate Usage IPs
First IP = Network Address + 1
Last IP = Broadcast Address - 1
Exercise
169.174.141.10/28
| Value | |
|---|---|
| IP Address | 169.174.141.10 |
| Subnet Mask | 255.255.255.240 |
| Network IP | 169.174.141.0 |
| Broadcast IP | 169.174.141.15 |
| Usable Hosts | 169.174.141.1 - 169.174.141.14 |
18.172.200.77/11
| Value | |
|---|---|
| IP Address | 18.172.200.77 |
| Subnet Mask | 255.224.0.0 |
| Network IP | 18.160.0.0 |
| Broadcast IP | 18.191.255.255 |
| Usable Hosts | 169.160.0.1 - 169.191.255.254 |