Services
DHCP
DHCP automates IPv4 address assignment using the DORA process. IOS routers can act as a DHCP server or client for lab and small-office scenarios.
What you need to know
- DORA process: Discover, Offer, Request, Acknowledge, all conducted via UDP broadcasts initially.
- DHCP client uses UDP port 68; DHCP server uses UDP port 67.
- excluded-address ranges must be configured before the pool to prevent the server from leasing reserved addresses like the default gateway.
- A DHCP pool defines the network, default-router, dns-server, and lease duration.
- Default lease time if unspecified is 24 hours (1 day 0 hours 0 minutes).
- ip dhcp excluded-address is configured globally, not inside the pool.
- A router interface can obtain its own address via ip address dhcp, making it a DHCP client.
Configuration commands
| Command | Mode | Purpose |
|---|---|---|
| ip dhcp excluded-address 10.10.10.1 10.10.10.10 | config | Reserves addresses the DHCP server should not lease. |
| ip dhcp pool LAN-POOL | config | Creates a DHCP pool and enters pool configuration mode. |
| network 10.10.10.0 255.255.255.0 | config-dhcp | Defines the network and subnet mask for the pool. |
| default-router 10.10.10.1 | config-dhcp | Sets the default gateway handed out to clients. |
| dns-server 8.8.8.8 | config-dhcp | Sets the DNS server address handed out to clients. |
| lease 7 | config-dhcp | Sets the lease duration to 7 days. |
| domain-name example.com | config-dhcp | Sets the DNS domain name option for clients. |
| ip address dhcp | config-if | Configures the interface to obtain its IP address from a DHCP server. |
| ip helper-address 10.10.10.1 | config-if | Forwards broadcast DHCP requests to a remote server (used with relay). |
Verify it
- show ip dhcp binding
- show ip dhcp pool
- show ip dhcp conflict
- show ip dhcp server statistics
- show ip interface brief
- debug ip dhcp server events
Common mistakes
- Forgetting ip dhcp excluded-address before creating the pool, causing the server to offer the router's own gateway address.
- Configuring the network statement with the wrong mask, shrinking or expanding the usable pool.
- Not disabling a conflicting DHCP service (ip dhcp conflict logging) when addresses appear double-assigned.
- Forgetting that pool configuration mode command names differ from global (network vs ip dhcp excluded-address).
- Leaving default-router unset, so clients get an address but no gateway.
Practise DHCP in a real CLI
Generated labs, graded against the simulated network state.