# DHCP leases, renewal and a relay

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/practice/mapping/lease

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/practice/mapping--lease/

Optional extension · no matching calculation in the two supplied papers

**Optional worked extension.** This calculation format was not found in the supplied December 2025 final or February 2026 re-exam. The numbers and practice marks are authored. Study the core concept and actual-paper questions first; this is extra explanation, not a TEE prediction.

**Definition.** A DHCP lease grants use of an address for a specified lifetime; renewal extends that permission, while a relay supports communication across subnets.

**Simple meaning.** Address permanent gift nahi: time-limited lease, phir renew.

## Understand it step by step

1. In ordinary DHCPv4 defaults, renewal begins at T1=50% of the lease and rebinding at T2=87.5%, unless the server supplies different values.
2. At T1 a client attempts renewal with its original server. At T2 it can seek renewal from any suitable server.
3. If the lease expires without extension, the client must stop using the leased address. Successful renewal restarts the relevant lease timing.
4. A relay forwards DHCP messages between subnets. It is not the same as a router forwarding every broadcast indiscriminately.

#### Practical example

For an 8-hour lease with default timers, T1 is 4 hours and T2 is 7 hours after lease acquisition. These defaults must not override explicit timers in the question.

### Practice question. What is a DHCP lease? Why is a DHCP relay used? [3 practice marks]

#### Answer

A lease permits a client to use its assigned address for a limited period and may be renewed. A relay conveys DHCP messages between a client subnet and a remote DHCP server, allowing centralized configuration without placing a server on every subnet.

#### Practical example

For an 8-hour lease with default timers, T1 is 4 hours and T2 is 7 hours after lease acquisition. These defaults must not override explicit timers in the question.

**Yaad rakho:** Lease = limited time; relay = subnet ke paar DHCP support.