Working

Working

DHCP works on the client-server model: a server that holds the IP Addresses distribution policy send a given configuration for a given period of time to a given client (typically, this client is a booting machine). The server is the base of all DHCP requests (he receives and replies to them), so it has to have a static IP configuration. In a local network it is then possible to have only one machine with a static IP address, i.e. the DHCP server.

The DHCP protocol is based on and extends BOOTP: it uses the same attribution mechanisms and messages.

When a machine boots, it has no IP configuration, so it should not be able to send a message on the network. However, there is a possibility, which is to use a specific broadcast address: to find and speak with a DHCP server, the booting machine will send broadcast messages with the 255.255.255.255 address. All the machines on the local network will then receive these broadcast messages, in particular the DHCP server. When the DHCP server receive these messages, it replies with other broadcast messages (with the same broadcast address) as long as the client gets its first configuration. The client must tell the server what configuration it finally keep. Once the client has sent an acknowledgement message, everything is done and the server keeps a track of transaction.

If the client already has an IP configuration, the dialogue is set without broadcast messages, but the exchange is still the same: negociation and then acknwolegement of a configuration by the client.

Leases

For resource optimisation on the network, IP addresses are allocated for a limited period of time. Allocating a configuration for a limited period of time is what we call a lease. A client that has its lease likely to become obsolete may ask the DHCP server for a renewal of its lease. In the same way, when a server notices that a lease is likely to end, it sends a reminder to the client to ask if it wishes to keep its lease. In both cases, if the server gets no reply, it declares that the given IP addresses are now free and may allocate them to another host straight after.

That is what make DHCP powerful: with leases, it is very convenient to optimize the allocation of IP addresses. Actually, the key of a perfect management is here: find out the right lease duration; if all IP addresses are allocated, then no more request would be satisfied.

On a local network where a lot of computers connect and deconnect often (a school of commercial premises), it is interesting to have short leases. On contrary, on a network with a lot of desktop machines, long leases are enough. Do not forget that DHCP mainly uses broadcast so noetwork resources may be limited by its mechanism; short leases mean a lot of broadcast messages...

Dynamic or not?

A DHCP server should provide dynamic IP addresses, that is to say a client can get successively get 2 different addresses. However, it is possible to ask it to provide a static IP address to a given host. This possibility should be used only as less as possible, otherwise the question is to know why there is a DHCP server! Anyway, this may be interesting for instance to give static addresses to DNS or TFTP servers.

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