220-1101 Question 56
Single answer2.1 Compare and contrast Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) ports, protocols, and their purposes.A technician is troubleshooting why new IP phones on a small office network are not receiving automatic network settings. The phones boot up, but they never obtain an IP address from the server. A packet capture shows the phones are sending broadcast traffic, but the firewall between the voice VLAN and the server VLAN only allows TCP ports 67 and 68. Which change should the technician make to restore service?
- A
Allow UDP ports 67 and 68 through the firewall
- B
Allow TCP port 53 through the firewall
- C
Allow UDP port 123 through the firewall
- D
Allow TCP port 443 through the firewall
Show answer and explanation
Correct answer: A
Explanation
This scenario tests practical understanding of TCP vs. UDP ports and their purposes. DHCP relies on UDP because it uses low-overhead, connectionless exchanges such as broadcast discovery messages when a client does not yet have an IP address. In a typical IPv4 DHCP process, the client uses UDP source port 68 and the server uses UDP source port 67. Allowing TCP 67/68 would not help because DHCP does not operate over TCP. This aligns with standard networking references and common best-practice firewall configurations for DHCP relay or inter-VLAN DHCP traffic. A+ candidates should recognize not only the port numbers, but also when UDP is required for infrastructure services such as DHCP.
- A. Correct.
Correct. DHCP uses UDP, not TCP. Specifically, DHCP server communication uses UDP port 67 and DHCP client communication uses UDP port 68. Because the phones are trying to obtain an IP address automatically, the firewall must permit UDP 67/68 for the DHCP exchange to succeed.
- B. Incorrect.
Incorrect. Port 53 is used for DNS, which can use both UDP and TCP depending on the query type, but DNS is not the service that initially assigns IP addressing information to the phones. A device must typically obtain an IP address first before normal DNS communication becomes relevant.
- C. Incorrect.
Incorrect. UDP port 123 is used by NTP for time synchronization. While IP phones often use NTP after joining the network, NTP does not provide IP addresses, subnet masks, default gateways, or other DHCP lease information.
- D. Incorrect.
Incorrect. TCP port 443 is used for HTTPS secure web traffic. Some phones or management systems may use HTTPS for provisioning or administration, but HTTPS is not the protocol used to request and receive an IP lease during startup.