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A user connects to the network and gets an IP. They can ping 8.8.8.8 but cannot open any website. Their IP is 10.10.10.87, gateway 10.10.10.1, DNS 8.8.8.8. They're on a corporate network where DNS should be an internal resolver, not Google. Something is handing out the wrong DNS server.
Three service problems, three diagnoses. DHCP is about getting an IP with correct options. NAT is about translating addresses for outbound traffic. DNS is about translating names to IPs. Each fails in characteristic ways, and each has a distinct diagnostic path.
The client's IP shows as having been leased by the server. Check the pool config: `show ip dhcp pool USERS`. Look at the `dns-server` and `default-router` options. If DNS is set to 8.8.8.8 but internal names require 10.10.99.53, clients get an IP but can't resolve internal names.
Static mapping — no active traffic. For dynamic traffic, you'd see entries with ports. If NAT isn't working at all, the first things to check: (1) inside/outside interfaces marked? (2) ACL for NAT matching the correct source? (3) `ip nat inside source list` referencing the right ACL and interface/pool? (4) `show ip nat statistics` for hits/misses.
DNS problems masquerade as connectivity issues. If IP works but names don't, you've already isolated to DNS. Check: `cat /etc/resolv.conf` for the resolver IP. Test with `nslookup` or `dig` against that resolver. Try against a known-good resolver (8.8.8.8) to isolate whether the issue is your resolver or the domain. For internal names, the client must be using an internal resolver that knows about them.
! Test DNS from Linuxdig @10.10.99.53 internal.example.com; <<>> DiG 9.18.18 <<>> @10.10.99.53 internal.example.com;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12345;; ANSWER SECTION:internal.example.com. 300 IN A 10.10.99.100! Compare with public resolver — should fail for internal namesdig @8.8.8.8 internal.example.com;; status: NXDOMAIN
DHCP gives out IPs and options — misconfiguration hands out wrong addresses. NAT translates for outbound traffic — misconfiguration blocks traffic. DNS resolves names — misconfiguration breaks browsing. Section 19 continues with wireless troubleshooting.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.