Subnetting is the skill of splitting one large address block into smaller networks that match real departments, VLANs or links. Work every example with paper first; then check with a calculator.
Key words in this topic
- Network address: first address of the subnet (host bits all 0).
- Broadcast address: last address (host bits all 1).
- Usable range: everything strictly between those two.
- VLSM: different subnet sizes inside one parent block, largest first.
- Default gateway: the router's address on this subnet; hosts send remote traffic there.
Worked example 1. Find the network, broadcast and host range for 192.168.10.77/26.
- /26 mask = 255.255.255.192. Block size = 256 - 192 = 64.
- Subnets in the last octet start at 0, 64, 128, 192.
- 77 lies between 64 and 128, so the network address = 192.168.10.64.
- Broadcast = next network minus 1 = 192.168.10.127.
- Usable range = 192.168.10.65 to 192.168.10.126 (62 hosts).
Worked example 2 (design). Divide 192.168.1.0/24 into 4 equal subnets.
- 4 subnets needs 2 bits (2^2 = 4), so the new prefix is /26 (mask 255.255.255.192), 62 hosts each.
| Subnet | Network | Usable range | Broadcast |
|---|---|---|---|
| 1 | 192.168.1.0/26 | .1 - .62 | .63 |
| 2 | 192.168.1.64/26 | .65 - .126 | .127 |
| 3 | 192.168.1.128/26 | .129 - .190 | .191 |
| 4 | 192.168.1.192/26 | .193 - .254 | .255 |
Worked example 3 (requirement-driven). A department needs 50 hosts. Smallest h with 2^h - 2 >= 50 is h = 6 (62), so use /26.
Practise: make up your own addresses, work them out by hand, then check with the subnet calculator in 4.4.