Use a Cisco Router as a DHCP server

Instead of having a dedicated DHCP server in a small office, you can use a Cisco router not only as a router but as a DHCP server as well.

1. Configuration

1.1 Exclude all IP addresses that you already have assigned or don’t want to be assigned via DHCP

Router(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.10

1.2 Create the DHCP pool

Router(config)# ip dhcp pool LAN_USERS

1.3 Define the network and subnet mask

Router(dhcp-config)# network 192.168.1.0 255.255.255.0

1.4 Configure the default-gateway

Router(dhcp-config)# default-router 192.168.1.1

1.5 Configure the DNS server

Router(dhcp-config)# dns-server 8.8.8.8

1.6 Configure the time lease

By default, the time that a device can use an IP before it has to renew it is 1 day on a Cisco router. That can be changed with the command:

Router(dhcp-config)# lease 7 0 0
Router(dhcp-config)# exit

3.Verification

To verify the clients that have received an IP address:

Router# show ip dhcp binding

UDP 67 -> server port
UDP 68 -> client port

DHCP options

• 3 -> Router
• 6 -> DNS Server
• 15 -> Domain Name
• 43 -> Tells the AP the IP address of the WLC
• 66 -> TFTP Server Name
• 150 -> TFTP Server IP address (Cisco Propietary)