DHCP (Dynamic Host Configuration Protocol)

Provides an ip address to every machine on the network and eliminates configuration errors.

IP Conflicts happen when the sam IP gets assigned to multiple machines inside the same network.

SCOP- a list of valid app addresses availabel for DHCP to assign.

Discover- when a device joins is til ask.

offer - ip address offered from the serfer

request - we take the address from the server

acknowledge - set’s the lease up.

ip adddress , subnet mask, default gatewa ip and DNS server are all configured.

dynamic assignemt

static assignemtn

if a device can’t reach the DHCP serer if dhcp fails it will default to the alternate configs. appipa or static ip address.

scope - subnet mask configured, default gateway configured, DNS server, and the lease time.

DHCP Relay Fowrards DHCP packeets between host and server. only needed if they are not on the same subnet or network. you can have a relay work across other server.

DHCP oppersates using UDP .

IP helper forwards several cinds of UDP broadcasts across the router. if the serve and requester are not on the same network will need the IP helper.

DHCP (Dynamic Host Configuration Protocol) (N10-009)

Purpose

Automatically provides IP addressing and configuration to devices.
Eliminates manual errors and IP conflicts.
Runs on UDP ports 67 (server) & 68 (client).

DHCP Process (DORA)

Discover – Client broadcasts to find a DHCP server.
Offer – Server replies with an available IP from its scope.
Request – Client requests that offered IP.
Acknowledge – Server confirms and leases the IP.

Configured Values (from DHCP scope):

IP Address
Subnet Mask
Default Gateway
DNS Server(s)
Lease time

Assignment Types

Dynamic Assignment → DHCP picks from the pool, lease can expire.
Static Assignment → Admin manually sets IP on device.
DHCP Reservation → IP is dynamically given, but always the same for a specific MAC.

Scopes

Scope = pool of valid IPs DHCP can hand out.
Includes subnet mask, gateway, DNS, lease time.

Failures

If DHCP unavailable →
- APIPA (169.254.x.x) self-assignment (Windows).
- Or use preconfigured alternate static IP.

DHCP Relay (a.k.a. IP Helper)

Forwards DHCP packets between client and server if not on the same subnet.
Router receives client’s broadcast → forwards as unicast to DHCP server.
IP Helper can also forward other UDP services (TFTP, DNS, NetBIOS, etc.).

✅ Exam Must-Knows

DHCP = UDP 67/68.
DORA = Discover → Offer → Request → Acknowledge.
Scope = list of assignable IPs + subnet/gateway/DNS/lease.
DHCP Relay/IP Helper needed across subnets.
APIPA = DHCP failure fallback.

⚡ Flashcards

Q: Which port does DHCP server listen on?

A: UDP 67.
Q: What does DORA stand for?

A: Discover, Offer, Request, Acknowledge.
Q: What happens if DHCP fails in Windows?

A: APIPA 169.254.x.x.
Q: What forwards DHCP across routers?

A: DHCP Relay / IP Helper.