IP Services · Topic 3 of 9
The Role of DHCP and DNS
Understand what DHCP and DNS each do on a network, how a host gets an address through DORA, how names resolve, and where the two services differ.
- 5 min
- Exam objective 4.3
- DHCP automates addressing. DNS resolves names.
- DORA is Discover, Offer, Request, Acknowledge.
- DHCP uses UDP 67 for the server and UDP 68 for the client.
What it is
DHCP gives devices their addressing, and DNS turns names into addresses.
They are often mentioned together, but they solve different problems.
DHCP (Dynamic Host Configuration Protocol) automatically supplies the settings a device needs. Without it, someone would type an address, mask, gateway and DNS server into every device.
A server hands these out from a scope or pool. Each set is leased for a set time, and the device must renew the lease to keep it.
A DHCP server can supply:
- an IP address and subnet mask
- a default gateway
- one or more DNS server addresses
- the lease length
- other options, such as a domain name
DNS (Domain Name System) is the network's phone book. People remember names, and devices need addresses.
DNS uses port 53. Ordinary queries use UDP. TCP is used for large replies and zone transfers.
How it works
A host gets its address from DHCP, then asks DNS to resolve the names it needs.
DHCP: the four steps
A host with no address follows a short conversation known as DORA.
| Step | Message | What happens |
|---|---|---|
| D | Discover | The client broadcasts: is there a DHCP server? |
| O | Offer | A server proposes an address and settings |
| R | Request | The client asks to use the offered address |
| A | Acknowledge | The server confirms the lease |
The client has no address yet, so it sends from 0.0.0.0 to the broadcast address. The server listens on UDP port 67 and the client on UDP port 68. The Request is also a broadcast, so other servers that made offers know the client chose someone else.
A Discover is a broadcast, and a router does not forward broadcasts. A server on another subnet needs a relay, covered in the DHCP client and relay topic.
Halfway through the lease, the client asks to renew, usually from the same server.
A client that never gets an answer to its first Discover often falls back to a link-local address starting with 169.254.
A DHCP server on a router
An administrator defines the scope, excludes addresses that must stay fixed, and can reserve an address for a device.
ip dhcp excluded-address 192.168.10.1 192.168.10.20
ip dhcp pool LAN10
network 192.168.10.0 255.255.255.0
default-router 192.168.10.1
dns-server 8.8.8.8
lease 7The lease value is in days by default. The excluded range keeps the first twenty addresses free for fixed devices.
DNS: how a name is resolved
The host checks its own cache first, then asks its configured resolver.
The authoritative server holds the official answer. The reply carries a TTL, which says how long everyone may cache it.
The record types to recognise:
| Record | Maps |
|---|---|
| A | Name to IPv4 address |
| AAAA | Name to IPv6 address |
| CNAME | Alias to another name |
| MX | Mail server for a domain |
| NS | Name server for a domain |
| PTR | Address back to a name |
DNS on a Cisco router
A router can resolve names you type at the command line.
ip domain lookup
ip name-server 8.8.8.8
ip domain name example.com
ip host LAB1 10.1.1.1ip name-serverpoints the router at a DNS server.ip hostcreates a local name-to-address entry.no ip domain lookupstops the long pause when you mistype a command and the router tries to resolve it as a hostname.
What to remember for the exam
Eight facts separate the two services.
- DHCP automates addressing. DNS resolves names.
- DORA is Discover, Offer, Request, Acknowledge.
- DHCP uses UDP 67 for the server and UDP 68 for the client.
- Discover is a broadcast, so a server on another subnet needs a relay.
- DHCP can supply a gateway and DNS server along with the address.
- Addresses are leased and must be renewed.
- DNS uses port 53. An A record is IPv4 and an AAAA record is IPv6.
- DNS answers carry a TTL that controls caching.
A ping to an IP address works but a ping to a name fails? Suspect DNS, not routing.
Common mistakes
These five confusions are the usual ones.
Mixing up the two services. A host can have a good DHCP address and still fail to browse if its DNS server is wrong.
Forgetting the broadcast limit. The fix is a relay on the router, not a bigger scope.
Skipping exclusions. If the pool includes the gateway or a printer's fixed address, DHCP can hand it to a second device and cause a conflict.
Thinking DHCP is permanent. A lease expires, so a long server outage eventually strips clients of addresses.
Confusing A and PTR. A goes from name to address. PTR goes from address to name.
Check yourself
Work through these three prompts.
What does each letter of DORA stand for, and which messages are broadcasts?
DORA is Discover, Offer, Request, Acknowledge. The Discover and the Request are broadcasts.
What is the difference between a DNS resolver and an authoritative server, and why does a TTL exist?
The resolver is the DNS server the host asks, while the authoritative server holds the official answer. The TTL says how long everyone may cache the reply.
Why might a host with a valid DHCP address still fail to reach a website by name?
DHCP and DNS are separate services, so the host's DNS server can be wrong even though its DHCP address is good.
Topic 3 of 9 in IP Services
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