NAT, PAT, and DHCP Configuration

Learning objectives

• Explain the purpose and scope of NAT, PAT, and DHCP Configuration.
• Identify important protocols, roles, fields, and terminology.
• Apply the concept to a practical network scenario.
• Use IOS XE commands to verify operational behavior.

1. Core concepts

DHCPv4 uses Discover, Offer, Request, Acknowledge to lease host configuration. A Cisco router can provide DHCP pools with network, default-router, DNS, and lease options. Exclude addresses reserved for gateways and infrastructure. If the DHCP server is on another subnet, configure ip helper-address on the client-facing Layer 3 interface.

2. How it works

NAT translates addresses as traffic crosses a boundary. Static NAT maps one inside local address to one inside global address. Dynamic NAT allocates from a pool. PAT overload allows many inside hosts to share one outside address using transport identifiers. Mark interfaces ip nat inside and ip nat outside and define the translation rule.

Visual walkthrough

3. Worked example

For a LAN 192.168.10.0/24, reserve .1–.20, serve the rest through DHCP, and translate inside traffic using the router WAN interface address with overload. Verify bindings, translations, counters, routing, and return traffic.

IOS XE commands and verification

Syntax and feature availability can vary by device family and software release. Confirm platform support before applying configuration.

ip dhcp excluded-address 192.168.10.1 192.168.10.20
ip dhcp pool USERS
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1
 dns-server 192.0.2.53
interface gi0/0/1
 ip nat inside
interface gi0/0/0
 ip nat outside
ip nat inside source list 10 interface gi0/0/0 overload
show ip dhcp binding
show ip nat translations

Common mistakes and troubleshooting

• Check physical and interface state before changing higher-layer settings.
• Verify both ends agree on addressing, VLAN, encapsulation, and protocol parameters.
• Use operational show commands; configured text alone does not prove a feature is working.
• Change one variable at a time and verify the result.

Knowledge check

Choose one answer per question. Submit to see why the correct choice is right and why every other option is wrong. These are original practice questions, not Cisco exam questions.