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Subnetting is one idea: take a network and split it by borrowing bits from the host portion. Every formula, every diagram, every exam question boils down to that. Miss the idea and the formulas feel arbitrary. Get the idea and you can derive the formulas on a napkin.
Take `192.168.10.0/24`. The first 24 bits are network; the last 8 are host. If you borrow 2 bits from the host portion, you get 2^2 = 4 subnets, each with 2^6 = 64 addresses.
Original: 192.168.10.00000000/24 (256 addresses, one subnet)Borrow 2: 192.168.10.00000000/26 → .0 – .63192.168.10.01000000/26 → .64 – .127192.168.10.10000000/26 → .128 – .191192.168.10.11000000/26 → .192 – .2554 subnets, 64 addresses each, 62 usable hosts each
Those borrowed bits become part of the network prefix. The mask lengthens from /24 to /26. That's all subnetting is — extending the network portion to subdivide the address space.
| Prefix | Host bits | Subnets (from /24) | Total addrs/subnet | Usable hosts |
|---|---|---|---|---|
| /25 | 7 | 2 | 128 | 126 |
| /26 | 6 | 4 | 64 | 62 |
| /27 | 5 | 8 | 32 | 30 |
| /28 | 4 | 16 | 16 | 14 |
| /29 | 3 | 32 | 8 | 6 |
| /30 | 2 | 64 | 4 | 2 |
The "−2" is for the network address and broadcast address. The network address identifies the subnet; the broadcast address reaches every host in it. Neither can be assigned to a host interface.
Given 10.20.0.0/16, carve subnets with at least 500 usable hosts each. 500 hosts needs 2^n - 2 >= 500, so n=9 (510 usable). Prefix = 32-9 = /23. Borrowed = 23-16 = 7 bits. Subnets = 2^7 = 128 x /23 blocks, each 512 addrs.
R1(config)# interface g0/0R1(config-if)# ip address 10.20.0.1 255.255.254.0R1(config-if)# no shutdown# 255.255.254.0 = /23, first subnet 10.20.0.0 - 10.20.1.255
Subnetting is mechanical once you see it. The trap is rushing the mask math and mistaking the boundary. Slow down, derive the mask, and the rest follows.
Next up: VLSM, which applies subnetting selectively instead of uniformly — the way real networks actually get designed.
Finish the lesson once you have worked through the material. This awards ★ 40 XP.