Dynamic Host Configuration Protocol
The Dynamic Host Configuration Protocol (DHCP) is a network protocol that is used to configure network devices so that they can communicate on an IP network. It involves clients and a server operating in a client-server model. In a typical personal home local area network (LAN), a router is the server[1] while clients are personal computers or printers. The router receives this information through a modem from an internet service provider which also operate DHCP servers where the modems are clients. The clients request configuration settings using the DHCP protocol such as an IP address, a default route and one or more DNS server addresses. Once the client implements these settings, the host is able to communicate on that internet.
One common effect of DHCP is to separate public (external) and private (internal) IP addresses. The ISP server may assign a globally unique external IP address to a home router and this IP address is used in internet communications. The router will then assign assign internal IP addresses to the clients connected to it, allowing the clients to broadcast only the external IP address.[2] This improves security by limiting access to devices and also helps to conserve IPv4 addresses. To directly access a device from outside the network, port forwarding may be used.
The DHCP server maintains a database of available IP addresses and configuration information. When the server receives a request from a client, the DHCP server determines the network to which the DHCP client is connected, and then allocates an IP address or prefix that is appropriate for the client, and sends configuration information appropriate for that client.
Because the DHCP protocol must work correctly even before DHCP clients have been configured, the DHCP server and DHCP client must be connected to the same network link. In larger networks, this is not practical. On such networks, each network link contains one or more DHCP relay agents. These DHCP relay agents receive messages from DHCP clients and forward them to DHCP servers. DHCP servers send responses back to the relay agent, and the relay agent then sends these responses to the DHCP client on the local network link.
DHCP servers typically grant IP addresses to clients only for a limited interval. DHCP clients are responsible for renewing their IP address before that interval has expired, and must stop using the address once the interval has expired, if they have not been able to renew it.
DHCP is used for IPv4 and IPv6. While both versions serve much the same purpose, the details of the protocol for IPv4 and IPv6 are sufficiently different that they may be considered separate protocols.[3]
Hosts that do not use DHCP for address configuration may still use it to obtain other configuration information. Alternatively, IPv6 hosts may use stateless address autoconfiguration. IPv4 hosts may use link-local addressing to achieve limited local connectivity.
Technical details
DHCP uses the same two ports assigned by IANA for BOOTP: destination UDP port 67 for sending data to the server, and UDP port 68 for data to the client. DHCP communications are connectionless in nature.DHCP operations fall into four basic phases: IP discovery, IP lease offer, IP request, and IP lease acknowledgement. These points are often abbreviated as DORA (Discovery, Offer, Request, Acknowledgement).
DHCP clients and servers on the same subnet communicate via UDP broadcasts, initially. If the client and server are on different subnets, a DHCP Helper or DHCP Relay Agent may be used. Clients requesting renewal of an existing lease may communicate directly via UDP unicast, since the client already has an established IP address at that point.
| Code | Name | Length | Notes |
|---|---|---|---|
| 50 | Requested IP Address | 4 octets | |
| 51 | IP Address Lease Time | 4 octets | |
| 52 | Option Overload | 1 octet | |
| 53 | DHCP Message Type | 1 octet | |
| 54 | Server Identifier | 4 octets | |
| 55 | Parameter Request List | minimum of 1 octet | |
| 56 | Message | minimum of 1 octet | |
| 57 | Maximum DHCP Message Size | 2 octets | |
| 58 | Renewal (T1) Time Value | 4 octets | |
| 59 | Rebinding (T2) Time Value | 4 octets | |
| 60 | Vendor class identifier | minimum of 1 octet | |
| 61 | Client-identifier | minimum of 2 octets | |
| 66 | TFTP server name | minimum of 1 octet | |
| 67 | Bootfile name | minimum of 1 octet |
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