• In this lesson, you will configure DHCP on a Cisco router so that devices in the LAN can automatically obtain their IP addressing information. You will also learn how to verify DHCP assignments and test the behavior of the DHCP server.

    DHCP configuration topology with PC1, SW1 and R1 on 192.168.1.0/24, linked to ISP via 203.0.113.0/30

    Figure 1 - DHCP Configuration Topology

    This is the network topology we will use throughout the lesson.

    Answer the question below

    Click 'Complete' to continue

    The LAN Side

    In this first task, you are the network administrator of the internal LAN.
    Your objective is to configure a DHCP pool that will automatically assign IP addresses to users in the 192.168.1.0/24 network.

    Before we configure anything, look at the topology:

    Topology highlighting R1 G0/0 (192.168.1.254) where the LAN DHCP pool for 192.168.1.0/24 is created

    Figure 2 - DHCP Topology

    The LAN has:

    • PC1 in the 192.168.1.0/24 network

    • SW1 acting as a Layer 2 switch

    • R1 acting as the default gateway (192.168.1.254)

    We want R1 to serve as the DHCP server for this network.

    Exclude Reserved IP Addresses

    In every network, some IP addresses must remain reserved.
    These addresses are used by devices that require a static IP address, such as:

    • routers

    • servers

    • printers

    To prevent IP conflicts, these addresses must be excluded from the DHCP scope.
    In our lab, the only reserved IP is the default gateway: 192.168.1.254.

    Here is how you define it:

    R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)# ip dhcp ?
  excluded-address  Prevent DHCP from assigning certain addresses
  pool              Configure DHCP address pools
  relay             DHCP relay agent parameters

R1(config)# ip dhcp excluded-address 192.168.1.254

    This ensures that 192.168.1.254 will never be assigned to a client.

    Create the DHCP Pool

    Now we create the DHCP pool that will assign IP addresses to clients.
    A DHCP pool is simply a named configuration where you define the settings the router will provide to DHCP clients.

    Before creating it, let’s look at the available DHCP commands:

    R1(config)# ip dhcp ?
  excluded-address  Prevent DHCP from assigning certain addresses
  pool              Configure DHCP address pools
  relay             DHCP relay agent parameters

    To create a pool, you must give it a name.
    The router guides you:

    R1(config)# ip dhcp pool ?
  WORD  Pool name

    We will name it LAN_POOL, because this pool will serve our internal LAN:

    R1(config)# ip dhcp pool LAN_POOL

    At this point, you enter DHCP pool configuration mode, where all DHCP options can be configured:

    R1(dhcp-config)# ?
  default-router  Default routers
  dns-server      Set name server
  domain-name     Domain name
  exit            Exit from DHCP pool configuration mode
  network         Network number and mask
  no              Negate a command or set its defaults
  option          Raw DHCP options

    Define the Network Range

    Next, you must tell the router which subnet this pool will serve.
    This step defines the network from which IP addresses will be assigned to DHCP clients.

    In our case, the LAN uses the 192.168.1.0/24 network, so we link the pool to this subnet:

    R1(dhcp-config)# network 192.168.1.0 255.255.255.0

    This tells the router that DHCP clients should receive addresses inside this subnet.

    Configure the Default Router

    DHCP clients also need a default gateway to reach external networks.
    This is the IP address of the router interface connected to the LAN.

    Here, the default gateway is 192.168.1.254, so we configure:

    R1(dhcp-config)# default-router 192.168.1.254

    This value will be automatically provided to clients when they obtain their IP address.

    Configure the DNS Server

    Clients must also know which DNS server to use to resolve domain names.
    We configure Google’s public DNS server:

    R1(dhcp-config)# dns-server 8.8.8.8

    This ensures that clients can immediately resolve domain names after receiving their configuration.

    Set the Domain Name (Optional but Recommended)

    Setting a domain name is optional, but it is common practice in enterprise networks and helps identify the LAN.

    We set the domain name to pingmynetwork.com:

    R1(dhcp-config)# domain-name pingmynetwork.com
R1(dhcp-config)# end

    Verify the Configuration

    Now that the DHCP pool is configured, you can verify it using:

    R1# show run | section dhcp
ip dhcp excluded-address 192.168.1.254
ip dhcp pool LAN_POOL
 network 192.168.1.0 255.255.255.0
 default-router 192.168.1.254
 dns-server 8.8.8.8
 domain-name pingmynetwork.com

    This output confirms that the DHCP pool is fully configured and ready to assign IP addresses to LAN clients.

    Answer the question below

    Look at the output of show run | section dhcp. Give the IP address that is excluded from the DHCP pool.