Lab 30: ISP-Style Edge — DHCP Client + PAT
Real edge routers often don't have a static public IP — they lease one from the ISP via DHCP. Configure the WAN interface as a DHCP client and overload NAT on it — exactly how small-office edges work. Difficulty: Intermediate · Time: ~25 min.
Lab objectives
- Set the WAN interface as a DHCP client
- Receive address + default route from the "ISP"
- Configure PAT overloading on the interface
- Verify LAN hosts reach beyond via the leased address
Topology & addressing
"ISP" router with a DHCP pool for 203.0.113.0/29 (simulating provider). Edge router: Gi0/1 to ISP (DHCP client), Gi0/0 = LAN 192.168.50.0/24 with PCs. A test server beyond the ISP.
Step-by-step configuration
Edge WAN: interface gi0/1ip address dhcpno shutdown | Leases address AND installs a default route via the ISP automatically |
access-list 1 permit 192.168.50.0 0.0.0.255ip nat inside source list 1 interface gi0/1 overload | PAT to whatever address the lease gave us |
Mark ip nat inside / ip nat outside as usual | Complete the NAT roles |
Verification
show ip interface brief — Gi0/1 shows its leased address. show ip route — an S* default route installed by DHCP. LAN PCs ping the far server; show ip nat translations shows them multiplexed behind the leased IP. Renew/release and watch NAT follow the new address — the beauty of overloading on the interface rather than a hardcoded IP.
Next lab: labs hub · test yourself: CCNA practice test.
Frequently asked questions
Why PAT on the interface instead of a specific IP?
Because the DHCP-leased address can change — overloading on the interface automatically uses whatever address is currently assigned.
Where does the default route come from?
The DHCP lease — ip address dhcp installs a default route toward the provider's gateway automatically.
Is this how home routers actually work?
Essentially yes — WAN DHCP client plus PAT on the WAN interface is the standard smalloffice/home edge pattern.
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