Dynamically assigning IP addresses

If there is no entry for the client in the configuration file, or if the client's entry does not contain an IP address, the DHCP server requests one from the Address Allocation Server. The AAS allocates an address from the address pool assigned to the client's subnet in the DHCP configuration file. If a subnet does not have an address pool configured, addresses cannot be allocated to clients on that subnet. Although there is no guarantee that the client will receive the same IP address it received last time, the AAS always attempts to do so. A DHCP client can also request a specific IP address. See ``IP address assignment priority''.

Every address offered by the DHCP server has either a fixed or infinite lease. See ``The lease for an assigned IP address''.

IP address assignment priority

The DHCP server dynamically assigns IP addresses using the following priorities:

1. Previous Address -- The DHCP server always assigns the address previously assigned to the client unless the address is unavailable.

2. Requested Address -- If the previous address is unavailable, the DHCP server assigns the addresses requested by the client. (A DHCP client can request a specific IP address by using the ``Requested IP Address'' option in the ``discover'' message. In most cases, this is the previous address.)

3. Arbitrary Address -- If the previous and requested address(es) are unavailable, the DHCP server assigns an arbitrary address from the configured pool.

The lease for an assigned IP address

When assigning an address pool to a subnet, you can also establish a default lease time and a maximum lease time for those addresses. The server uses the default lease time unless the client requests a specific lease time using the ``IP Address Lease Time'' option in either a ``discover'' message or a ``request'' message. If the client requests a lease time greater than the maximum, the server offers the maximum lease time.

Both the default and maximum leases can be configured to a maximum of 136 years. There is no minimum lease. Both leases can also be specified as infinite.

Clients can request an extension of the address lease. In addition to default and maximum lease times, you can configure the lease renewal and rebind time for the addresses in the pool. The DHCP server sends the lease renewal and rebind times to the client in the ``ack'' message. The server defines these values as percentages of the lease. The default renewal time in 50%, suggesting the client request a renewal when the lease is half over. The rebind time is the point at which the server suggests the client broadcast a renewal request. This is a second renewal attempt and accommodates multiple DHCP servers that may be sharing the address pool. The default is 90%. See ``Configuring subnet option sets'' for information about specifying leases and renewal information.

To prevent an address from being assigned to more than one client at the same time, DHCP temporarily reserves the address until the client accepts or rejects it. You can configure this initial reservation time in the DHCP configuration database as a server parameter. See ``Specifying DHCP server parameters''. After the client accepts the address, the DHCP server notifies AAS that the address has been leased. If the AAS is not notified, the address can be reallocated by the address server after the reservation time expires.

A leased IP address becomes available again when either the DHCP server receives a ``release'' message from the client or the lease expires. To account for the possibility of drift between the clocks of the server and client, you can specify a Lease Padding parameter which extends the length of the lease. The default is 1% of the lease. The unpadded lease is known to the client. The DHCP server knows the padded lease so preventing the server from thinking that the lease expired before the client does. See ``Specifying DHCP server parameters''.