6 Core Network Services
Learn how DHCP, DNS, and NAT provide client configuration, name resolution, and connectivity between networks, and how to verify each service.
Provide client configuration with
leases clients an IP address and can also provide a subnet mask, default gateway, server, and lease duration. The client configuration must match the intended subnet.
Set scopes, relays, and leases
A must match the client subnet. Exclude the gateway, servers, and other statically addressed devices from the dynamic range to avoid address conflicts.
In this Cisco IOS-style example, the scope serves 192.168.10.0/24, reserves .1 through .30 for infrastructure, and supplies clients with a gateway and server:
A server on a different subnet will generally need a configured on the client-facing router interface because broadcasts do not normally cross routers. On Cisco IOS, relay configuration commonly uses ip helper-address on that interface.
Check the server’s pool, active leases, and conflicts with:
On a client, renew the lease and inspect its assigned address, mask, gateway, and server. On Windows, use ipconfig /release, ipconfig /renew, and ipconfig /all. A valid-looking address alone is not enough: confirm that it belongs to the intended subnet and that the gateway and values are correct.
Resolve names with
is a distributed, hierarchical database. servers publish records in zones; client resolvers query servers and can cache answers. Common records include A (name to IPv4 address), AAAA (name to IPv6 address), (alias), and (address to name).
An internal zone might contain an mapping files.example.test to 192.168.10.20. Create and maintain records on the organization’s authoritative platform. Configuring a router to use an upstream server configures that router as a resolver, not as an .
Configure and test
Clients need the IP address of a reachable , often delivered through . On Cisco IOS, the router’s own resolver can be pointed at servers using commands such as:
Use addresses appropriate to the network; the example addresses are illustrative. Check name resolution from a client with nslookup files.example.test or dig files.example.test. To distinguish a server or record problem from an incorrect client resolver setting, query a particular server directly, for example with nslookup files.example.test 192.168.10.10. Confirm both the returned address and that the client can reach it. Cisco IOS show hosts displays configured name servers and cached host information.
Translate addresses with and
changes IP addresses as traffic passes between networks. , also called overload, lets multiple inside IPv4 hosts share one outside address by distinguishing their connections with transport-layer ports. is not a substitute for a firewall or access-control policy.
This Cisco IOS-style example marks the LAN as inside and the WAN as outside, then translates traffic from the LAN using the WAN interface’s address. The WAN interface shown obtains its address by ; use the correct interface names and WAN configuration for the actual installation.
Generate traffic from a LAN client to an external destination, then check the router’s translations and counters:
A translation entry should appear for active traffic that matches the access list. If none appears, check that the correct interfaces are marked inside and outside, the access list matches the client’s source subnet, the router has a working route to the WAN, and the client is actually sending traffic.
Verify the services in sequence
Use this sequence to narrow down connectivity problems:
Check addressing: Compare the client’s IP address, subnet mask, gateway, and server with the intended scope.
Check local connectivity: Ping the default gateway. If this fails, investigate the client link, VLAN, address, or gateway before testing external services.
Check : Query a known name and, if necessary, query the configured server directly. Compare the answer with the expected record.
Check routed connectivity and : Test an external IP address, then generate a new connection and inspect translations and counters.
Narrow down the failure: If an external IP is reachable but names are not, investigate . If the client cannot reach its gateway, is not yet the first issue to troubleshoot.
provides addresses and network options; resolves names through records published in zones; and / translates traffic between address realms, often allowing multiple private IPv4 clients to share one public-facing address. Reliable operation depends on checking the actual client configuration as well as service-specific leases, records, translations, and counters. Correct interface roles, subnet values, routes, and address reservations are essential.