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ip dhcp pool

IP DHCP POOL

The ip dhcp pool command allows us to create a DHCP pool on a Cisco IOS device. This command is available on most Layer 3 devices running Cisco IOS and is used to configure the DHCP server service on these devices.

The format of the command is as follows:

ip dhcp pool <pool-name>

DHCP pool configuration parameters

Some of the commonly used parameters used when configuring a DHCP pool on a Cisco IOS device are as follows:

  • network (defined by a network address and subnet mask - this is the IP subnet for which the DHCP pool is being created.)
  • default-router (the IP address of the default gateway for the subnet)
  • dns-server (a list of dns server addresses for DHCP clients to use - a total of 8 server addresses are supported in each pool.)
  • domain-name (the DNS suffix to assign to DHCP clients)
  • lease (the length of time for which the DHCP lease is valid - specified as "Days Hours Minutes")
  • option (specific DHCP options)

In the following example, we create a dhcp pool for the subnet 192.168.0.0/24:

RTR-1 Console
RTR-1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
RTR-1(config)#ip dhcp pool Vlan10-computers
RTR-1(dhcp-config)#network 192.168.0.0. 255.255.255.0
RTR-1(dhcp-config)#default-router 192.168.0.1
RTR-1(dhcp-config)#dns-server 8.8.8.8 1.1.1.1
RTR-1(dhcp-config)#domain-name home.lab
RTR-1(dhcp-config)#lease 7 0 0
RTR-1(dhcp-config)#

A DHCP pool was created with the following details :

  • The name of the pool is set to Vlan10-computers. (This helps us identify the purpose of this pool.)
  • The pool was created to assign client IP addresses from the subnet 192.168.0.0 /24.
  • Clients receiving their IP configuration from this pool will be assigned a default gateway address of 192.168.0.1.
  • Clients receiving their IP configuration from this pool will be assigned two DNS servers: 8.8.8.8 and 1.1.1.1.
  • Clients receiving their IP configuration from this pool will be assigned the domain suffix home.lab.
  • IP addresses leased out from this pool will be leased out for a period of 7 days, 0 hours and 0 minutes.

Caveat: The router above will assign IP addresses from this pool (when it receives a DHCP Discover message), as long as one of the following two conditions are met:

  • The router receives a DHCP Discover message on an interface that is configured with an IP address from the subnet 192.168.0.0/24; or,
  • The router receives a DHCP Discover message that includes a Gateway IP address value that falls within the subnet 192.168.0.0/24. (This would happen if the DHCP Discover message is forwarded by a DHCP Relay Agent.)

Most often, when we find that a Cisco IOS device is not assigning IP addresses from one of the pools configured on it, potential reasons would be:

  • The dhcp service has been disabled (using the command no service dhcp); or,
  • There is no Layer 3 interface configured with an IP address that belongs to the subnet for which the pool has been configured.

Are we able to exclude one or more IP addresses from a DHCP pool? Yes, we are. This can be done using the command ip dhcp excluded-address.

DHCP, DNS and NAT are covered extensively in the following interactive course: Course 6 - IP Services. The course features visual, animated explanations and hands-on interactive labs.

Want to test your networking skills with hands-on configuration and troubleshooting questions? Try out the Workshops section of this website here: Workshops (Login required).

Read how to use the device simulators on this website here: Workshops - General Instructions