How DHCP Works: DORA, Scopes, Leases, and Troubleshooting
DHCP automatically gives clients the IP settings they need to join a network. This guide explains DORA, scopes, leases, relay agents, reservations, and troubleshooting.
How DHCP Works: DORA, Scopes, Leases, and: Table of Contents
1. The DORA Process
DHCP commonly follows Discover, Offer, Request, and Acknowledge.
2. Scopes, Leases, and Options
A scope defines available addresses. Options provide gateway, DNS, domain, and other client settings.
- Default gateway option.
- DNS server option.
- Lease duration.
- Reservations for known devices.
3. Cisco DHCP Server Example
Cisco routers can provide DHCP in labs and small branches. Exclude static addresses before creating the pool.
- 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
4. DHCP Relay
DHCP broadcasts do not cross routers by default. Use ip helper-address on the client VLAN gateway when the server is remote.
- interface Vlan10
- ip helper-address 10.10.10.5
5. Troubleshooting DHCP
Check VLAN, scope exhaustion, exclusions, relay, ACLs, and incorrect gateway or DNS options.
- ipconfig /all
- ipconfig /renew
- show ip dhcp binding
- show ip dhcp pool
How DHCP Works: DORA, Scopes, Leases, and: Frequently Asked Questions
What does DHCP stand for?
Dynamic Host Configuration Protocol.
What is DORA?
Discover, Offer, Request, and Acknowledge.
What ports does DHCP use?
DHCP uses UDP 67 for server and UDP 68 for client.
How DHCP Messages Move Through the Network
A new client normally has no usable IPv4 address, so its DHCP Discover is sent as a local broadcast. A server on the same VLAN can answer directly. When the server is on another subnet, the Layer 3 gateway must act as a DHCP relay. On Cisco IOS, ip helper-address is configured on the client-facing routed interface or SVI, not on the server-facing interface. The relay converts the broadcast into a routed request and records the client subnet so the server selects the correct scope.
The server Offer proposes an address, mask, lease time and options. The client Request identifies the offer it wants, which also tells other servers that their offers were not selected. The Acknowledge completes the lease. A negative acknowledgement can occur when the requested address is not valid for the current subnet or when the server cannot honor the request.
Designing Scopes and Options
Build one scope for each client subnet and exclude infrastructure addresses before clients begin requesting leases. The default gateway option must point to the router interface in that client VLAN. DNS server options should reference resolvers the client can actually reach. Choose a lease duration that matches device behavior: stable office devices can use longer leases, while guest or event networks often need shorter leases so unused addresses return to the pool.
Reservations are useful for printers, phones and appliances that should retain a predictable address while remaining centrally managed. A reservation ties an address to a client identifier or MAC-related value. It is different from configuring a static address directly on the endpoint, and the exact identifier used should be verified on the DHCP server.
A Repeatable DHCP Troubleshooting Method
- Confirm the client link, VLAN membership and switchport state.
- Check whether the client received an APIPA address in
169.254.0.0/16, which indicates that the DHCP exchange failed. - Verify the scope has free addresses and that exclusions do not consume the available range.
- Check the relay address and routed reachability between the gateway and server.
- Inspect ACLs, firewalls and DHCP snooping policies for blocked UDP 67 or 68 traffic.
- Compare the assigned gateway, mask and DNS values with the intended subnet plan.
A packet capture is especially useful because it shows which DORA message is missing. If Discover is visible but no Offer returns, focus on the server, relay path or scope. If an Offer returns but the client never completes the Request, investigate competing servers, client state or Layer 2 filtering. Continue with the router configuration guide, subnetting tutorial, and network troubleshooting practice test.
How DHCP Works: DORA, Scopes, Leases, and: Tags and Keywords
DHCP, DORA process, DHCP scope, DHCP lease, DHCP relay, ip helper-address, DHCP troubleshooting, CCNA DHCP