- Early routing protocols like RIP v1 only supported Fixed Length Subnet Masking where all subnets had to be the same size. So no /28 and /27 in the same network
- VLSM does support different size subnets on the same network.
- Start with the largest segment... at the start of the address space, and work down to the smallest segment. Example problem below:
NY Eng 200.15.10.1 - .30 (network .0, broadcast .31) 255.255.255.224 /27
Boston Eng 200.15.10. 33 - .62 (network .32, broadcast .63) 255.255.255.224 /27
NY Sales 200.15.10.65 - .78 (network .64, broadcast .79) 255.255.255.240 /28
Boston Sales 200.15.10.81 - .94 (network .80, broadcast .95) 255.255.255.240 /28
NY - Bost 200.15.10.97 - .98 (network .96, broadcast .99) 255.255.255.252 /30
Bonus: The Magic Number Method
- If address 60.15.10.75 / 28, and you need to find the network address, broadcast address, and range of valid IP addresses?
- So the subnet mask is 255.255.255.240
- Subtract the value in the subnetted octet from 256: 256-240 = 16
- The network address go up in 16's: 0-15, 16-31, 32-47, 48-63, 64-79......
- Network add. =60.15.10.64; broadcast = 60.15.10.79; valid hosts from .65 -.78
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