TCP/IP and Data Communications Administration Guide


The DHCP/BOOTP server calculates a client's IP address lease using information contained in the dhcptab and dhcp_network databases. The server consults the LeaseTim and LeaseNeg symbols in the dhcptab database, and the Flags and Lease fields of the chosen dhcp_network database record.

The server first examines the Flags field for the identified dhcp_network record. If the PERMANENT or BOOTP flag is on, then the client's lease is considered permanent.

If the PERMANENT flag is not on, then the server checks to see if the client's lease, as represented by the Lease field in the dhcp_network record, has expired. If it has not, the server checks to see if the client has requested a new lease. If the LeaseNeg symbol has not been included in the client's dhcptab parameters, then the client's requested lease extension is ignored, and the lease is set to be the time remaining as shown in the Lease field.

If the LeaseNeg symbol has been included, the server extends the client's lease to the value it requested, provided this requested lease is less than or equal to the current time plus the value of the client's LeaseTim dhcptab parameter. If the client's requested lease is greater than policy allows (the policy is the value of LeaseTim), then the client is given a lease equal to the current time, plus the value of LeaseTim. If LeaseTim is not set, then the default LeaseTim value is one hour.