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Understanding Dynamic Host Configuration Protocol
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The Dynamic Host Configuration Protocol (DHCP) for IPv4 service automates the assignment of IPv4 addresses, subnet masks, gateways, and other IPv4 networking parameters. This is referred to as dynamic addressing.
The alternative to dynamic addressing is static addressing. When using static addressing, the network administrator manually enters IP address information on hosts.
When a host connects to the network, the DHCP server is contacted, and an address is requested. The DHCP server chooses an address from a configured range of addresses called a pool and assigns (leases) it to the host.
On larger networks, or where the user population changes frequently, DHCP is preferred for address assignment. New users may arrive and need connections; others may have new computers that must be connected.
Rather than use static addressing for each connection, it is more efficient to have IPv4 addresses assigned automatically using DHCP.
DHCP can allocate IP addresses for a configurable period of time, called a lease period. The lease period is an important DHCP setting, When the lease period expires or the DHCP server gets a DHCPRELEASE message the address is returned to the DHCP pool for reuse.
Users can freely move from location to location and easily re-establish network connections through DHCP.
Various types of devices can be DHCP servers. The DHCP server in most medium-to-large networks is usually a local, dedicated PC-based server. With home networks, the DHCP server is usually located on the local router that connects the home network to the ISP.
The figure depicts an ISP DHCP server connected to the Internet with three ISP routers labelled ISP1, ISP2, ISP#. Each ISP router is connected to a different network. ISP1 connects a wireless antenna to a mobile worker who is the DCHP client. ISP2 is connected to a corporate network router that connects to a corporate LAN with its own local DHCP server connected to a switch connected to six DHCP clients.
ISP3 is connected to a wireless DHCP server for a Home and Small Business network with the three DHCP clients connected.
Many networks use both DHCP and static addressing. DHCP is used for general-purpose hosts, such as end-user devices. Static addressing is used for network devices, such as gateway routers, switches, servers, and printers.
DHCP for IPv6 (DHCPv6) provides similar services for IPv6 clients. One important difference is that DHCPv6 does not provide a default gateway address. This can only be obtained dynamically from the Router Advertisement message of the router.
DHCP Operation
As shown in the figure, when an IPv4, DHCP-configured device boots up or connects to the network, the client broadcasts a DHCP discover (DHCPDISCOVER) message to identify any available DHCP servers on the network.
A DHCP server replies with a DHCP offer (DHCPOFFER) message, which offers a lease to the client. The offer message contains the IPv4 address and subnet mask to be assigned, the IPv4 address of the DNS server, and the IPv4 address of the default gateway. The lease offer also includes the duration of the lease.
The figure shows a protocol ladder with a DHCP client on one side and a DHCP client on the other. The DHCP client sends a DHCPDISCOVER message to the DHCP Server. The DHCP server sends a DHCPOFFER message to the DHCP client. The DHCP client sends a DHCPREQUEST message in response to the DHCPOFFER from the DHCP server. THE DHCP server sends a DHCPACK message back to the DHCP client. The process is called DORA.
The client may receive multiple DHCPOFFER messages if there is more than one DHCP server on the local network. Therefore, it must choose between them and sends a DHCP request (DHCPREQUEST) message that identifies the explicit server and leases offer that the client is accepting. A client may also choose to request an address that it had previously been allocated by the server.
Assuming that the IPv4 address requested by the client, or offered by the server, is still available, the server returns a DHCP acknowledgement (DHCPACK) message that acknowledges to the client that the lease has been finalized. If the offer is no longer valid, then the selected server responds with a DHCP negative acknowledgement (DHCPNAK) message.
If a DHCPNAK message is returned, then the selection process must begin again with a new DHCPDISCOVER message being transmitted. After the client has the lease, it must be renewed prior to the lease expiration through another DHCPREQUEST message.
The DHCP server ensures that all IP addresses are unique (the same IP address cannot be assigned to two different network devices simultaneously). Most ISPs use DHCP to allocate addresses to their customers.
DHCPv6 has a set of messages that is similar to those for DHCPv4. The DHCPv6 messages are SOLICIT, ADVERTISE, INFORMATION REQUEST, and REPLY.
DHCP Message Format
The DHCPv4 message format is used for all DHCPv4 transactions. DHCPv4 messages are encapsulated within the UDP transport protocol. DHCPv4 messages that are sent from the client use UDP source port 68 and destination port 67. DHCPv4 messages sent from the server to the client use UDP source port 67 and destination port 68. The structure of the DHCPv4 message is shown below.
Operation (OP) Code – Specifies the general type of message. A value of 1 indicates a request message; a value of 2 is a reply message.
Hardware Type – Identifies the type of hardware used in the network. For example, 1 is Ethernet, 15 is Frame Relay, and 20 is a serial line. These are the same codes used in ARP messages.
Hardware Address Length – Specifies the length of the address.
Hops – Controls the forwarding of messages. Set to 0 by a client before transmitting a request.
Transaction Identifier – Used by the client to match the request with replies received from DHCPv4 servers.
Seconds – Identifies the number of seconds elapsed since a client began attempting to acquire or renew a lease. Used by DHCPv4 servers to prioritize replies when multiple client requests are outstanding.
Flags – Used by a client that does not know its IPv4 address when it sends a request. Only one of the 16 bits is used, which is the broadcast flag. A value of 1 in this field tells the DHCPv4 server or relay agent receiving the request that the reply should be sent as a broadcast.
Client IP Address – Used by a client during lease renewal when the address of the client is valid and usable, not during the process of acquiring an address. The client puts its own IPv4 address in this field if and only if it has a valid IPv4 address while in the bound state; otherwise, it sets the field to 0.
Your IP Address – Used by the server to assign an IPv4 address to the client.
Server IP Address – Used by the server to identify the address of the server that the client should use for the next step in the bootstrap process, which may or may not be the server sending this reply. The sending server always includes its own IPv4 address in a special field called the Server Identifier DHCPv4 option.
Gateway IP Address – Routes DHCPv4 messages when DHCPv4 relay agents are involved. The gateway address facilitates communications of DHCPv4 requests and replies between the client and a server that are on different subnets or networks.
Client Hardware Address – Specifies the physical layer of the client.
Server Name – Used by the server sending a DHCPOFFER or DHCPACK message. The server may optionally put its name in this field. This can be a simple text nickname or a DNS domain name, such as dhcpserver.netacad.net.
Boot Filename – Optionally used by a client to request a particular type of boot file in a DHCPDISCOVER message. Used by a server in a DHCPOFFER to fully specify a boot file directory and filename.
DHCP Options – Holds DHCP options, including several parameters required for basic DHCP operation. This field is variable in length. Both client and server may use this field.
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