[BiPolar_Nut]

From the DHCP RFC (RFC2131):

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4.4.5 Reacquisition and expiration

The client maintains two times, T1 and T2, that specify the times at which the client tries to extend its lease on its network address. T1 is the time at which the client enters the RENEWING state and attempts to contact the server that originally issued the client's network address. T2 is the time at which the client enters the REBINDING state and attempts to contact any server. T1 MUST be earlier than T2, which, in turn, MUST be earlier than the time at which the client's lease will expire.

To avoid the need for synchronized clocks, T1 and T2 are expressed in options as relative times [2].

At time T1 the client moves to RENEWING state and sends (via unicast) a DHCPREQUEST message to the server to extend its lease. The client sets the 'ciaddr' field in the DHCPREQUEST to its current network address. The client records the local time at which the DHCPREQUEST message is sent for computation of the lease expiration time. The client MUST NOT include a 'server identifier' in the DHCPREQUEST message.

Any DHCPACK messages that arrive with an 'xid' that does not match the 'xid' of the client's DHCPREQUEST message are silently discarded. When the client receives a DHCPACK from the server, the client computes the lease expiration time as the sum of the time at which the client sent the DHCPREQUEST message and the duration of the lease in the DHCPACK message. The client has successfully reacquired its network address, returns to BOUND state and may continue network processing.

If no DHCPACK arrives before time T2, the client moves to REBINDING state and sends (via broadcast) a DHCPREQUEST message to extend its lease. The client sets the 'ciaddr' field in the DHCPREQUEST to its current network address. The client MUST NOT include a 'server identifier' in the DHCPREQUEST message.

Times T1 and T2 are configurable by the server through options. T1 defaults to (0.5 * duration_of_lease). T2 defaults to (0.875 * duration_of_lease). Times T1 and T2 SHOULD be chosen with some random "fuzz" around a fixed value, to avoid synchronization of client reacquisition.

A client MAY choose to renew or extend its lease prior to T1. The server MAY choose to extend the client's lease according to policy set by the network administrator. The server SHOULD return T1 and T2, and their values SHOULD be adjusted from their original values to take account of the time remaining on the lease.

In both RENEWING and REBINDING states, if the client receives no response to its DHCPREQUEST message, the client SHOULD wait one-half of the remaining time until T2 (in RENEWING state) and one-half of the remaining lease time (in REBINDING state), down to a minimum of 60 seconds, before retransmitting the DHCPREQUEST message.

If the lease expires before the client receives a DHCPACK, the client moves to INIT state, MUST immediately stop any other network processing and requests network initialization parameters as if the client were uninitialized. If the client then receives a DHCPACK allocating that client its previous network address, the client SHOULD continue network processing. If the client is given a new network address, it MUST NOT continue using the previous network address and SHOULD notify the local users of the problem.

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T2 may be 24 hours or 2 weeks (or 1 minute or 23.85 years). T1 is typically ½ that time, which is when your DHCP device (DSL/Cable router) will try to renew it's IP lease and keep the same address.

You could possibly keep the same dynamic IP for years in many/most common DHCP situations.