SY0-301 · Question #811
After a new firewall has been installed, devices cannot obtain a new IP address. Which of the following ports should Matt, the security administrator, open on the firewall?
The correct answer is B. 68. DHCP clients listen on UDP port 68 to receive IP address assignments from a DHCP server, so this port must be open on the firewall for IP allocation to function.
Question
After a new firewall has been installed, devices cannot obtain a new IP address. Which of the following ports should Matt, the security administrator, open on the firewall?
Options
- A25
- B68
- C80
- D443
How the community answered
(43 responses)- A5% (2)
- B91% (39)
- C2% (1)
- D2% (1)
Why each option
DHCP clients listen on UDP port 68 to receive IP address assignments from a DHCP server, so this port must be open on the firewall for IP allocation to function.
Port 25 is used by SMTP for sending email and has no role in IP address assignment.
The DHCP protocol (RFC 2131) uses UDP port 67 on the server side and UDP port 68 on the client side. When a client broadcasts a DHCPDISCOVER message, the server responds to the client on port 68. If the firewall blocks port 68, the DHCP reply from the server never reaches the client, preventing the client from obtaining an IP address. Opening port 68 allows the DHCP offer and acknowledgment to complete successfully.
Port 80 is used by HTTP for unencrypted web traffic and is unrelated to DHCP.
Port 443 is used by HTTPS for encrypted web traffic and has no involvement in the DHCP address assignment process.
Concept tested: DHCP client port 68 for IP address assignment
Source: https://www.rfc-editor.org/rfc/rfc2131
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