DHCP client

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

A DHCP client is a network interface or device that requests configuration parameters such as an IP address from a DHCP server.

A DHCP client is a host or network interface that requests an IP address and related parameters from a DHCP server. The server leases an address and can provide a netmask, default gateway, and DNS server, avoiding error-prone manual configuration. section 4.1 section 4.4

Use the DORA sequence to remember how a client obtains a lease:

• DISCOVER: broadcasts from 0.0.0.0 to 255.255.255.255 to locate DHCP servers. section 4.1.1 section 4.4
• OFFER: a server proposes an IP address and other configuration parameters. section 4.1.1
• REQUEST: the client accepts an offer, normally broadcasting its selection. section 4.1.1 section 4.4
• ACK: the server confirms and finalizes the lease, enabling network communication. section 4.1.1 section 4.4

DHCP uses UDP port 67 for the server and UDP port 68 for the client. DHCP is stateful because the server tracks leased addresses. Unlike ordinary end hosts, routers and switches usually use static addresses, but an ISP-facing router interface commonly acts as a DHCP client. section 4.1.1 section 4.1.3

Configure a Cisco IOS interface as a DHCP client

Enter interface configuration mode, apply "ip address dhcp", and enable the interface with "no shutdown". IOS then reports the assigned address; "show ip interface" identifies the address as DHCP-derived, and "show ip route" can show an automatically installed default route through the ISP. That DHCP-learned route is displayed with the code S, although it was not manually configured. section 4.1.3 section 4.1.3

Figure 4.2 Leasing an IPv4 address via DHCP (the DORA process): (1) DISCOVER from client to server, (2) OFFER from server to client, (3) REQUEST from client to server, (4) ACK from server to client.

Follow the four-message IPv4 exchange from client DISCOVER through server ACK. section 4.1.1

Figure 4.5 R1 is a DHCP client of the ISP. (1) Configure G0/1 to learn its IP address via DHCP. (2) DORA exchange between R1 and the ISP router. (3) R1 configures the learned IP on G0/1, and adds a default route via the ISP router.

Trace an ISP-connected router interface as it learns an address and installs a default route. section 4.1.3

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes

Explains DORA, UDP ports, stateful leases, IOS configuration, and verification, but stops short of DHCP server and relay configuration.

Examples

Uses PC clients, Cisco IOS router interfaces, ISP connections, DORA exchanges, and DHCP verification output.

What it assumes

Assumes recognition of IP addresses, netmasks, default gateways, DNS servers, interfaces, and routing-table output.

What you can do after

Operate/configure a Cisco IOS interface as a DHCP client and verify its learned address and default route.

How it connects

Connects DHCP clients with DHCP servers, dynamic IP addresses, default gateways, DNS, UDP, IPv4, and static IP addresses.

liveBookmanning.com

Read Acing the CCNA Exam: Volume 2 Advanced Networking and Security on liveBook Get the book at manning.com

Related concepts

• IP addressproduces · An IP address is a numerical address used by the Internet Protocol to identify a network interface and support packet delivery across networks.
• dhcpuses · The Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns devices IP addresses and related configuration information.
• centralized DHCP serveruses · A centralized DHCP server is a DHCP server that provides address and configuration services to clients across multiple networks from a central location.
• default routeproduces · A default route is a routing-table entry used when no more specific route matches a destination.
• clientkind of · A client is a person, organization, device, or software component that receives or uses a product, service, resource, or functionality provided by another entity.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 2 Advanced Networking and Security. The section links point to the parts of the book it draws on.