Dynamic Host Configuration Protocol

R3zk0n · October 2, 2025

Network
Contents

    How does a computer know what its IP configuration is?

    • Most likely, a computer received its IP configuration from a DHCP (Dynamic Host Configuration Protocol) server.
    • Not only did the server give the PC an IP address, but it also told the PC where the default gateway was and more than likely how to find a DNS server.
    • A computer will receive its IP configuration in one of two ways, statically (manually set) or dynamically (through a service like DHCP).
    • Static address assignment works fine for very small and stable networks, but quickly becomes unwieldy and error prone as the network grows

    Default Gateway

    • A default gateway is the node in a computer network using the Internet protocol suite that serves as the forwarding host (router) to other networks when no other route specification matches the destination IP address of a packet.

    Static IP addressing

    • The administrator assigns an IP number and subnet mask to each host in the network.
    • Each network interface that is going to be available to connect to the network requires this information.
    • The administrator assigns a default gateway location and DNS server location to each host in the network
    • These are required if access outside of the network is going to be allowed (default gateway) and human friendly naming conventions are allowed to find network resources (DNS server).
    • Each time a change is made (e.g., a new default gateway is established) each IP configuration on each host must be updated.

    Dynamic IP addressing

    • The administrator configures a DHCP server to handie the assigning process, which automates the process.
    • The DHCP server listens on a specific port for IP information requests.
    • Once it receives a request, the DHCP server responds with the required information

    How DHCP works

    • Upon boot up, a PC that is configured to request an IP configuration sends a DHCP discovery packet
    • The discovery packet is sent to the broadcast address: 255.255.255 255 67 (UDP port 67).
    • The DHCP server receives the discovery packet and responds with an offer packet
    • The offer packet is sent to the MAC address of the computer using UDP port 68.
    • The computer receives the offer packet from the DHCP server and returns a request packet (requesting the proper IP configuration) to the DHCP server.
    • Once the DHCP server receives the request packet, it sends back an acknowledgement packet, which contains the required IP configuration information.
    • Upon receipt of the acknowledgement packet, the PC changes its IP configuration to reflect the information received

    What is a broadcast address?

    • A broadcast address is an IP address that is used to target all systems on a specific subnet network instead of single hosts. In other words broadcast address allows information to be sent to all machines on a given subnet rather than to a specific machine.

    Components and Processes of DHCP

    Ports Used

    • PC sends discovery packet to 255.255 255 255.67.
    • DHCP sends offer packet to the PC’s MAC address on port 68.

    Address Scope

    • Administrator configures the IP address range with one that is available to be handed out.

    Address Reservations

    • Administrator reserves specific IP addresses to be handed out to specific MAC addresses. These are used for devices that should always have the same IP address (eg., servers and routers).
    • Allows for these addresses to be changed from a central location instead of having to log in to each device separately.

    Leases

    • Configuration parameters are only good for a specified amount of time.
    • Leases are configured by the administrator.

    Options

    • Default gateway location
    • DNS server addresses (there can be more than one).
    • Time server addresses.

    Preferred IP configuration

    • A PC can have a preferred IP address.
    • The administrator can configure the DHCP server to either honor the preference or ignore it.

    Do DHCP servers need to reside on the same network segment?

    • Broadcast transmissions cannot pass through a router. If there is not a DHCP server on the local network segment, the router can be configured to be a DHCP relay.
    • When a DHCP relay (which can also be called an IP helper) receives a discovery packet from a node, it will forward that packet to the network segment on which the DHCP server resides.
    • This allows for fewer DHCP servers on any given network, reducing maintenance that administrators need to perform.

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