4.5 - DHCP Relay

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Indiana Institute of Technology *

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2000

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Information Systems

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Feb 20, 2024

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Question 1: Which port is a DHCP Discover packet sent out on when a device first connects to a network? 68 80 53 67 Explanation When a device first connects to a network, it sends out a DHCP Discover packet. This packet is a UDP broadcast sent out on port 67 to all devices on the network, asking if there's a DHCP server available. Port 68 is used by the DHCP relay agent to send out a DHCP broadcast packet to a client. Port 80 is used for HTTP. Port 53 is used by DNS. Question 2: Which port does the relay agent use when sending DHCP information back to the client? 53 67 68 80 Explanation When a DHCP server receives the DHCP Discover packet from the relay agent, the server sends back a DHCP Offer packet. The relay agent puts the DHCP information into a DHCP broadcast packet and sends it to the client on UDP port 68 on the local network. When a device first connects to a network, it sends out a DHCP Discover packet. This packet is a UDP broadcast sent out on port 67 to all the devices on the network, asking if there's a DHCP server available. Port 80 is used for HTTP. Port 53 is used by DNS. Question 3:

Which of the following is the last step in a DHCP request process that uses a DHCP relay agent? The DHCP server receives the DHCP discover packet and sends a DHCP offer packet to the relay agent. The server returns the DHCP ACK packet which the relay agent forwards to the client. The relay agent sends the unicast packet to the DHCP server located on another network. The DHCP relay agent receives the broadcast message and converts the packet into a unicast message. Explanation The server returning the DHCP ACK packet and the relay agent forwarding the packet to the client is the last step in the process. The following are the remaining steps (in order) of the process: 1. The DHCP relay agent receives the broadcast message and converts the packet into a unicast message. The relay agent uses its IP address as the source IP. 2. The relay agent sends the unicast packet to the DHCP server located on another network. 3. The DHCP server receives the DHCP discover packet and sends a DHCP offer packet to the relay agent. 4. The relay agent puts the DHCP information into a DHCP broadcast packet and sends it to the client on UDP port 68 on the local network. 5. The client processes the DHCP information and sends the DHCP request packet to the relay agent which then forwards it on to the DHCP server. Question 4: What is the first thing a device does when it connects to a network? Sends a DHCP Request packet. Sends a DHCP ACK packet. Sends a DHCP Offer packet. Sends a DHCP Discover packet. Explanation When a device first connects to a network, it sends out a DHCP Discover packet. This packet is a UDP broadcast sent out on port 67 to all the devices on the network, asking if there's a DHCP server available.

All the other answers occur later in the DHCP process. Question 5: On a typical network, what happens if a client attempts to receive DHCP configuration from a DHCP server that's located on a different subnet? The router drops the DHCP request. The client will not send a DHCP request. The DHCP request is automatically forwarded to the server. The request needs to be manually forwarded to the server. Explanation When a DHCP server is on a different network, the router drops the packet because routers do not forward broadcast packets to other networks. DHCP requests are not automatically forwarded to another network. You need to configure a DHCP relay agent for this. Clients still send out DHCP requests, even if they do not know where the DHCP server is. DHCP requests are not manually forwarded. Question 6: Which of the following does the DHCP relay agent use to tell the DHCP server which pool of addresses to use? GIADDR Subnet mask DHCP ACK DHCP scope Explanation The relay agent adds the Gateway IP Address (GIADDR) field to the DHCP request that defines the gateway, or network, that the client belongs to. The GIADDR tells the DHCP server the pool of addresses to use when assigning an IP address to the client. The DHCP scope is configured on the DHCP server. The scope defines the DHCP information for the network. The DHCP scope is not used by the relay agent to tell the DHCP server which pool of addresses to use. The DHCP ACK is the final step in the DHCP process. The DHCP ACK is not used by the relay agent to tell the DHCP server which pool of addresses to use.

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The subnet mask is configured on the DHCP scope and defines which network the client belongs to. The subnet mask is not used by the relay agent to tell the DHCP server which pool of addresses to use. Question 7: Which of the following would a device use to receive NTP configuration from a NTP server that's located on a different network? UDP relay agent NTP relay agent NTP helper IP helper Explanation The IP helper performs the same actions as a DHCP relay agent for other UDP-based protocols, such as NTP. As with DHCP relay, the IP helper forwards to servers that are located on different networks or subnets. The process that takes place is identical when forwarding UDP packets. Question 8: Which of the following is a UDP-based protocol that uses an IP helper to perform the same actions as a DHCP relay agent? NTP IIS SMB TCP Explanation The implementation of an IP helper is a lot like the DHCP relay. Our client sends out a request for a UDP protocol such as NTP (Network Time Protocol). The following are not UDP-based protocols:  IIS (Internet Information Services) is a commonly used web server that runs on Windows.  TCP (Transmission Control Protocol) enables application programs and computing devices to exchange messages over a network.  An SMB (Server Message Block) server provides shared resources, like folders, files, and printers, to network hosts and relies on TCP.

Question 9: When a DHCP relay agent receives a broadcast message requesting an available DHCP server, the DHCP uses an IP address as the source IP address. Which of the following is the IP address used by the DHCP relay agent? The DHCP server's IP address The requesting device's IP address The gateway's IP address Its own IP address. Explanation When a DHCP relay agent receives a broadcast message requesting an available DHCP server, the DHCP relay agent uses its own IP address as the source IP address. The device, gateway and DHCP server addresses are not used. Question 10: Which of the following protocols does DHCP use when it sends out IP configuration? TCP SMTP IP UDP Explanation DHCP uses User Datagram Protocol (UDP) when sending out configuration information to clients. Transmission Control Protocol (TCP) provides services that ensure accurate and timely delivery of network communications between hosts. This does not verify if an IP address is currently in use on the network. Internet Protocol (IP) allows devices to communicate across different networks. This does not verify if an IP address is currently in use on the network. Simple Mail Transfer Protocol (SMTP) is used to send outgoing emails.

4.5 - DHCP Relay

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