Setting up a DHCP Server on a Cisco Router

Nerd Cafe

Scenario:

You are configuring a Cisco router as a DHCP server to dynamically assign IP addresses to client devices on a local network.

1. Understanding DHCP

DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses, subnet masks, default gateways, and other parameters to network devices. A Cisco router can function as a DHCP server, providing these settings to clients.

2. Practical Scenario

• You are setting up a small office network.

• The router will act as a DHCP server.

• The local network uses the 192.168.1.0/24 subnet.

• The router's interface (FastEthernet0/0) will be the gateway 192.168.1.1.

• The DHCP server should assign addresses from 192.168.1.100 to 192.168.1.200.

• Some devices (like a printer) will need static IP reservations.

3. Network Topology

Topology

4. Configuration Steps

Step 1: Enter Global Configuration Mode

Connect to the router via CLI (Console, SSH, or Telnet), then enter global configuration mode:

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#

Step 2: Exclude Reserved IP Addresses

Reserve specific IP addresses that should not be assigned by DHCP (e.g., for the router, printers, and servers):

R1(config)#ip dhcp excluded-address 192.168.1.1 192.168.1.10

• 192.168.1.1 is the router’s IP.

• 192.168.1.2 – 192.168.1.10 are reserved for static devices.

Step 3: Create a DHCP Pool

Define a DHCP pool named OFFICE_NETWORK:

R1(config)#ip dhcp pool OFFICE_NETWORK
R1(dhcp-config)#

Step 4: Define the Network and Subnet Mask

Specify the subnet for DHCP clients:

R1(dhcp-config)#network 192.168.1.0 255.255.255.0

Step 5: Set Default Gateway

Assign the router’s IP as the default gateway:

R1(dhcp-config)#default-router 192.168.1.1

Step 6: Define the DNS Server

Specify a DNS server (e.g., Google’s 8.8.8.8):

R1(dhcp-config)#dns-server 8.8.8.8

Step 7: Configure the Lease Time

Set a lease duration (e.g., 24 hours):

R1(dhcp-config)#lease 1 0 0

1 0 0 means 1 day, 0 hours, 0 minutes.

Step 8: Exit the DHCP Configuration

R1(dhcp-config)#exit
R1(config)#

Step9: Assign IP address to fastEthernet 0/0 (Connected to PC1)

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#no shutdown

Step 9: PC1 Configuration

PC1> ip dhcp
DORA IP 192.168.1.11/24 GW 192.168.1.1

PC1> show ip

NAME        : PC1[1]
IP/MASK     : 192.168.1.11/24
GATEWAY     : 192.168.1.1
DNS         : 8.8.8.8
DHCP SERVER : 192.168.1.1
DHCP LEASE  : 86395, 86400/43200/75600
MAC         : 00:50:79:66:68:00
LPORT       : 20008
RHOST:PORT  : 127.0.0.1:20009
MTU         : 1500

PC1>

Step 10: Printer (Static IP)

• Manually set IP: 192.168.1.2, Gateway: 192.168.1.1.

• Ensure it doesn’t request DHCP.

PC2> ip 192.168.1.2/24 192.168.1.1
Checking for duplicate address...
PC2 : 192.168.1.2 255.255.255.0 gateway 192.168.1.1

PC2> show ip

NAME        : PC2[1]
IP/MASK     : 192.168.1.2/24
GATEWAY     : 192.168.1.1
DNS         :
MAC         : 00:50:79:66:68:01
LPORT       : 20010
RHOST:PORT  : 127.0.0.1:20011
MTU         : 1500

PC2>

5. Verifying DHCP Configuration

1. Check the DHCP Configuration

R1#show ip dhcp pool

Pool OFFICE_NETWORK :
 Utilization mark (high/low)    : 100 / 0
 Subnet size (first/next)       : 0 / 0
 Total addresses                : 254
 Leased addresses               : 0
 Pending event                  : none
 1 subnet is currently in the pool :
 Current index        IP address range                    Leased addresses
 192.168.1.1          192.168.1.1      - 192.168.1.254     0
R1#

This shows the active DHCP pool and available IP addresses.

2. View DHCP Bindings (Leased IPs)

R1#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type
                    Hardware address/
                    User name
192.168.1.11        0100.5079.6668.00       Mar 23 2025 05:54 AM    Automatic
R1#

This displays assigned IP addresses and MAC addresses.

7. Troubleshooting Commands

• Check if the DHCP service is running:

R1#show running-config | inclu
R1#show running-config | include dhcp
ip dhcp excluded-address 192.168.1.1 192.168.1.10
ip dhcp pool OFFICE_NETWORK
R1#

• Debug DHCP process:

R1#debug ip dhcp server events
DHCP server event debugging is on

Keywords

DHCP, Cisco Router, IP Address, Subnet Mask, Default Gateway, DNS Server, Lease Time, Excluded Address, DHCP Pool, Network Configuration, IP Binding, DHCP Reservation, Dynamic IP, Static IP, Show Commands, Debugging, Packet Tracer, Network Automation, Router Configuration, CCNA, سیسکو