Ports blocked on Comcast's network

Updated 4/8/2014 9:43:48 PM by Comcast Expert


For the protection of the network and our customers, Comcast Business Internet blocks certain ports. Learn which ports are blocked and why.

The ports listed below are blocked to protect against common viruses and worms, malicious intruders, and other security risks. These ports are blocked to protect our customers and the integrity of the Comcast Network; these blocks cannot be lifted.

Port Transport Protocol How and why it's blocked

Inbound only, blocked by default.

Used to obtain dynamic Internet protocol (IP) address information from Comcast's dynamic host configuration protocol (DHCP) server that assigns IP addresses to customer computers.

DHCP ports can be used for malicious attacks, such as obtaining access to a computer or network and its devices.


Inbound and outbound, blocked by default.

Allows file sharing over networks.

When improperly configured, they can expose critical system files or give full file system access (run, delete, copy) to any malicious intruder connected to the network.


Inbound and outbound, blocked by default.

Security risks; vulnerable to attacks, exploits, and worms (such as Sasser and Ninder).


Inbound and outbound, blocked by default.

Vulnerable to malicious route updates, which provide several attack possibilities.


Inbound only, blocked by default.

Multiple vulnerabilities (viruses, worms, DoS attacks).

In very rare situations, port 25 will be blocked by Comcast Customer Security Assurance on a per customer basis (blocked at the modem) and not across the network. This block will be preceded by an email and letters to the billing address. In this case, you can use secured port 587 for sending email. Get instructions to configure your email client to send via a secured port.

If you are running a mail server please contact Comcast Customer Security Assurance at 1-877-807-6580 for more information on this block.

Port Transport Protocol How and why it's blocked

Inbound and outbound, not blocked by default.

We may apply a sending block, which does not interrupt Comcast webmail service. However, it will prevent email programs or clients (e.g., Outlook Express) from sending email.

An unsecured port that can be used to send spam.

Customers may be advised by our Security Assurance team to switch their modem connection to a secured port that requires authentication (such as port 587).

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