Core
Subnetting
Subnetting divides a network into smaller segments by borrowing host bits for additional network bits. Fast, accurate subnetting is a core CCNA skill tested heavily on the exam.
What you need to know
- Number of subnets = 2^n where n is bits borrowed; number of usable hosts = 2^h - 2 where h is remaining host bits.
- Block size (increment) = 256 - value of the interesting octet in the mask, e.g. /26 gives block size 64.
- Common masks: /24=255.255.255.0, /25=.128, /26=.192, /27=.224, /28=.240, /29=.248, /30=.252.
- A /30 subnet yields 2 usable hosts, ideal for point-to-point WAN links.
- A /31 subnet (RFC 3021) yields 2 usable addresses with no network/broadcast, for point-to-point links only.
- To find the subnet of an address, round down to the nearest multiple of the block size in the relevant octet.
- Broadcast address is one less than the next subnet boundary.
Configuration commands
| Command | Mode | Purpose |
|---|---|---|
| ip address 192.168.1.65 255.255.255.192 | config-if | Configure an address inside a /26 subnet. |
| ip address 172.16.0.1 255.255.255.252 | config-if | Configure a /30 point-to-point link address. |
| ip address 10.0.0.1 255.255.255.254 | config-if | Configure a /31 point-to-point link address. |
| show ip route connected | exec | Display connected subnets learned from configured interfaces. |
| ip subnet-zero | config | Allow use of the zero subnet (enabled by default on modern IOS). |
| no ip subnet-zero | config | Disallow use of the zero subnet (legacy behavior). |
Verify it
- show ip interface brief
- show ip route
- show interfaces | include Internet address
- ping <address>
Common mistakes
- Miscounting borrowed bits when calculating the number of subnets or hosts.
- Forgetting to subtract 2 for network and broadcast addresses in usable host counts.
- Placing two device addresses in different subnets on the same physical link.
- Using a /30 or /31 mask on a multi-access LAN segment that needs more hosts.
- Mixing up the block size increment with the mask value itself.
Practise Subnetting in a real CLI
Generated labs, graded against the simulated network state.