Reef Basics - How to Configure a Cisco Router with a DHCP WAN IP and Internal DHCP Pool with NAT

How to Configure Cisco ISR4331 with Dynamic WAN IP, NAT, and Internal DHCP (LAN 192.168.0.0/24): Reef Telecom Guide

Example Topology

• WAN interface: Automatically receives IP via DHCP (GigabitEthernet0/0/0)

• LAN subnet: 192.168.0.0/24 on GigabitEthernet0/0/1

• Internal gateway: 192.168.0.1

1. Configure the WAN (Outside) Interface for DHCP

• interface GigabitEthernet0/0/0

• description WAN / Internet (DHCP)

• ip address dhcp

• ip nat outside

• no shutdown

2. Configure the LAN (Inside) Interface

• interface GigabitEthernet0/0/1

• description LAN

• ip address 192.168.0.1 255.255.255.0

• ip nat inside

• no shutdown

3. Exclude Reserved LAN Addresses from DHCP

• ip dhcp excluded-address 192.168.0.1 192.168.0.50

4. Configure the Internal DHCP Pool

• ip dhcp pool LAN-POOL

• network 192.168.0.0 255.255.255.0

• default-router 192.168.0.1

• dns-server 8.8.8.8

• lease 7

5. Set Up NAT Overload (PAT) for Internet Access

• access-list 1 permit 192.168.0.0 0.0.0.255

• ip nat inside source list 1 interface GigabitEthernet0/0/0 overload

6. Ensure a Default Route for Outbound Traffic

• ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0/0

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How Does This Work?

• The WAN interface automatically gets an IP from your ISP with DHCP.

• LAN devices get internal addresses (192.168.0.51-192.168.0.254) from your router’s DHCP server.

• NAT allows all your internal devices to use your WAN IP to access the internet.

• The default route directs all external traffic to the WAN connection.

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Useful Show Commands

• show ip interface brief

• show ip dhcp binding

• show ip nat translations

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FAQ

• What if my ISP uses PPPoE or requires a username/password?
  Contact Reef Telecom for those advanced setups.

• Can I reserve IPs for servers/internal printers?
  Yes—add them to the excluded DHCP range.

• Does this work for cable/DSL/fiber ISPs?
  Yes—any service where the WAN interface gets a DHCP lease.

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Config Command Summary for Copy/Paste:

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