DHCP
DHCP is how networks hand out IP addresses automatically instead of typing one into every device by hand. This hub gathers the step-by-step guides, command cheat sheet, and graded labs you need to configure a Cisco router as a DHCP server and to relay DHCP across subnets with ip helper-address.
New to this? Start with the explainer: What Is DHCP and How Does It Work?
Dynamic Host Configuration Protocol (DHCP) leases IP addresses, subnet masks, default gateways, and DNS servers to hosts automatically through the four-step DORA exchange: Discover, Offer, Request, and Acknowledge. On the CCNA 200-301 blueprint it sits in the IP Services domain, where you are expected to configure and verify DHCP relay and the DHCP client role (exact section weighting varies by exam revision, so treat percentages as approximate). Because nearly every production network assigns addresses this way, DHCP is one of the highest-value practical skills the exam covers.
The first guide, configuring a DHCP server on a Cisco router, covers the address pool itself. You create a named scope with 'ip dhcp pool', define the 'network' it serves, and set the options clients need: 'default-router' for the gateway, 'dns-server', 'domain-name', and an optional 'lease' time (the default lease is one day). The step beginners skip is reserving addresses. 'ip dhcp excluded-address' is a global command, not a command inside the pool, and it keeps the server from leasing addresses already in use by the router interface, static servers, or printers. Miss it and you get duplicate-address conflicts.
The second guide, DHCP relay with ip helper-address, solves the cross-subnet problem. A DHCP Discover is a broadcast, and routers do not forward broadcasts, so a client on one subnet cannot reach a server on another. Configuring 'ip helper-address <server-ip>' on the interface facing the clients converts those broadcasts into a unicast (directed) forward toward the server, and by default it relays eight UDP services (including DNS, TFTP, and the BOOTP/DHCP ports 67 and 68), not DHCP alone. This is the same relay behavior whether the server is a dedicated appliance or another router.
Seen together, the landscape is small but connected: a router can BE the DHCP server, or it can RELAY to a server elsewhere, and larger networks usually do both at once. Verification ties the pieces together. 'show ip dhcp binding' lists the leases that have been handed out, 'show ip dhcp pool' shows address usage, and 'show ip dhcp conflict' surfaces addresses you forgot to exclude. Knowing which show command answers which question is exactly what separates a working configuration from a lucky guess.
Master this topic in three passes. First, read the two guides to understand the flow end to end: the server pool and its options, then the relay. Second, keep the command cheat sheet next to you so the pool-versus-global distinction (options like 'default-router' live inside the pool, while 'ip dhcp excluded-address' is entered at global config) becomes muscle memory. Third, build and grade the hands-on labs: stand up a pool, break it by forgetting to exclude the gateway, then rescue a client stranded behind a router with 'ip helper-address'. The bundle sequences these so each lab grades whether addressing actually works, not merely whether the commands were typed.
Step-by-step guides
Follow these to configure it yourself, command by command.
- How to Configure a DHCP Server on a Cisco Router (Step by Step)Step-by-step guide to configuring a Cisco IOS DHCP server: exclude addresses, build the pool, verify bindings, and relay DHCP with ip helper-address.
- How to Configure DHCP Relay (ip helper-address) on Cisco (Step by Step)Configure DHCP relay with ip helper-address so clients lease from a DHCP server on another subnet. Step-by-step Cisco IOS setup, verification, and common fixes.
Practice on real Cisco IOS
Build and grade hands-on Cisco Modeling Labs — the only way it sticks.
Frequently asked questions
What is the difference between a DHCP server and a DHCP relay on a Cisco router?
The DHCP server, configured with 'ip dhcp pool', owns the address scope and actually leases addresses, gateways, and DNS settings to clients. A DHCP relay, configured with 'ip helper-address' on the client-facing interface, owns no addresses; it simply forwards clients' broadcast requests to a DHCP server that lives on a different subnet. A single router can act as a server, as a relay, or as both at the same time.
Which order should I learn these DHCP topics in?
Start with the router-as-DHCP-server guide (pool, default-router and dns-server options, and excluded addresses), because it teaches what a DHCP conversation actually contains. Then move to the DHCP relay guide, since ip helper-address only makes sense once you know what it is forwarding and why a broadcast cannot cross the router on its own. Finish by building and grading the labs so the commands stick.
Why does a Cisco DHCP server hand out an address that is already in use?
By default the pool will lease every address inside its 'network' statement, including the router's own gateway address and any statically assigned servers or printers. You reserve those with 'ip dhcp excluded-address', which is entered at global config so the server never offers them. Skipping this step is the usual cause of duplicate-address conflicts, which you can confirm with 'show ip dhcp conflict'.
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