Dec 10

Explained The DHCP, Simple and Easy


DHCP Definition:

In a hyper-connected world, the ability of a device to get connected with other devices quickly and easily is critical. Each device must have a unique IP address to access a TCP/IP based network and its resources. Without the presence of DHCP, the IP address for a new computer is configured manually and if it is removed then the IP address has also to be reclaimed manually.

The DHCP (Dynamic Host Configuration Protocol) is a network protocol used on TCP/IP network to assign an IP address and other information automatically to each host so they can communicate efficiently with other endpoints. Including IP address, the DHCP also provides subnet mask, default gateway address, DNS address and other relevant configuration parameters. Manually assigning IP address to devices might cause two hosts to possess the same IP address which likely leads to errors and most users sometimes are unable to locate the IP address information on a computer. So, automating the process simplifies the IP Address management for users and network administrators.

Components of DHCP:

  1. DHCP Server – The DHCP server is a network device that maintains a pool of IP addresses and related configuration parameters. It leases IP address to DHCP enabled client as to when requested. Because the IP addresses are dynamically assigned, so once they are not in use they are automatically returned to the pool of allocation.
  2. DHCP Client – The DHCP client is the device that receives IP address configuration from a DHCP server.
  3. DHCP Relay – The DHCP relay is a router or host that acts as a middle man between DHCP client and server. It listens to the messages from clients broadcasted on the network and forwards it to the configured DHCP server. The server then replies back with messages which the relay agent passes them unto the client. This is used to centralize the DHCP server instead of using them in each subnet.

How DHCP works:

The following steps explain how it works:

  1. When the DHCP client which is a computer or device wants to connect to the network, it asks its server or router for an IP address. This broadcast message sent by the client is called “DHCP Discovery” request.
  2. The DHCP server receives the request, it sends a “DHCP Offer” message which contains information as IP address leased to the client, subnet mask of the network segment, IP address of the default gateway, DNS domain name, DNS server address or addresses, and TFTP address or addresses. Since the client doesn’t have an IP address, the DHCP server broadcasts the DHCP Offer message to the local network.
  3. All the devices in the network receive the DHCP Offer broadcast message but only the client who had requested the IP address accepts the message and the rest of the hosts discard the message. This happens because the DHCP Offer message contains a parameter called as client id which is actually the mac address of the client device. The server gets this information from the DHCP Discover message which was sent by the host. When host finds the its MAC address in the client ID it knows that this DHCP Offer message is meant for it. Then the client sends a “DHCP Request” message to the server to know that it wants to use that IP configuration. But if the IP configuration doesn’t match the client configuration, then the host can respond with a “DHCP Decline” message. In that case, the DHCP server offers another IP configuration.
  4. When DHCP server receives the DHCP Request from the client, it responds with a “DHCP ACK” message as an acknowledgment to the client confirming that the client can use the offered IP configuration. In some cases, the server may reply with a “DHCP NACK” message indicating that the offer is no longer valid and the client needs to request an IP configuration again. This situation usually occurs when the client takes too long to reply to the DHCP Offer message from the server.

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