DHCP and Network Address Translation

Overview

Recall questions on the purpose of DHCP and how it works, what else the server supplies, why addresses are leased, and what NAT does.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is the purpose of DHCP?

   Model answer

   Its purpose is to configure devices automatically when they join a network, so that IP addresses do not have to be set by hand on every machine, and so that the available addresses can be shared out among devices as they come and go rather than being permanently tied to particular machines.

2. 2

   Describe how DHCP works when a device joins a network.

   Model answer

   A DHCP server holds a pool of addresses it is allowed to hand out. When a device joins the network it broadcasts a request for configuration, since it has no address yet and does not know where the server is. A DHCP server hears the broadcast and offers one of the free addresses from its pool. The device accepts the offer and the server records the allocation. Along with the IP address the server supplies the subnet mask, the address of the default gateway and the addresses of the domain name servers to use. The allocation is a lease for a fixed period, and the device must renew it before it expires or the address returns to the pool for reuse.

3. 3

   What does a DHCP server supply besides the IP address?

   Model answer

   The subnet mask, so the device knows which part of its address is the network identifier; the address of the default gateway, so it knows where to send traffic for other networks; and the addresses of the domain name servers to use for resolving names. It also supplies the lease duration.

4. 4

   Why are addresses leased for a limited period rather than given out permanently?

   Model answer

   Because devices come and go. A laptop or phone that leaves the network permanently would otherwise keep an address that nobody could reuse, and the pool would gradually empty. A lease means an address returns to the pool automatically if it is not renewed, so a network with far more registered devices than addresses can still function as long as they are not all present at once.

5. 5

   Give two advantages of DHCP over configuring addresses by hand.

   Model answer

   Any two of: it removes the labour of configuring every machine individually, which on a large network would be substantial; it eliminates the human errors that manual configuration invites, particularly assigning the same address twice, which would break both machines; it lets addresses be reused, so a limited pool can serve a larger number of devices; it allows visiting devices to join and work with no configuration at all; and a change to the gateway or name server addresses can be made once at the server and propagates to every device as leases renew.

6. 6

   When would you deliberately not use DHCP for a device?

   Model answer

   For a device that other machines need to find at a fixed address — a server, a router, a network printer, a name server. If its address changed, everything configured to reach it would break. Such devices are given a static address, or the DHCP server is configured to always reserve the same address for them.

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