When the DHCP server is considering dynamically allocating an IP address to a client, it first sends an ICMP Echo request (a ping) to the address being assigned. It waits for a second, and if no ICMP Echo response has been heard, it assigns the address. If a response is heard, the lease is abandoned, and the server does not respond to the client. The lease will remain abandoned for a minimum of abandon-lease-time seconds.

If a there are no free addressses but there are abandoned IP addresses, the DHCP server will attempt to reclaim an abandoned IP address regardless of the value of abandon-lease-time (default 24h).

This ping check introduces a default one-second delay in responding to DHCPDISCOVER messages, which can be a problem for some clients. The default delay of one second may be configured using the ping-timeout parameter. The ping-check configuration parameter can be used to control checking - if its value is false, no ping check is done.