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Ports blocked on Comcast's network

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
0 TCP N/A

Inbound and outbound, blocked by default.

Port 0 is a reserved port, which means it should not be used by applications. Network abuse has prompted the need to block this port.

68 UDP BOOTP/DHCP

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.

135
-139
TCP/UDP NetBIOS

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.

161
-162
TCP/UDP SNMP

Inbound and outbound, blocked by default.

SNMP is vulnerable to reflected amplification distributed denial of service (DDoS) attacks.

445 TCP MS-DS-SMB

Inbound and outbound, blocked by default.

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

520 TCP/UDP RIP

Inbound and outbound, blocked by default.

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

1080 TCP SOCKS

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.

 

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
25 TCP SMTP

Inbound and outbound, not blocked by default.

We may apply a sending block, which 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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New Member

PORT 21 blocked COMCAST BUSINESS -- NOT Xfinity

Port 21  Blocked Comcast Business NOT Xfinity  Repeat NOT Xfnity

 

Does NOT allow FTP

 

Have been working on this for over a weekl

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Official Employee

Re: PORT 21 blocked COMCAST BUSINESS -- NOT Xfinity

Hello, @CarlStreet, please click on my name and use the send message option to send a private message. If you can include your name, account number, and service address we can take a closer look. Most of the time Port Forwarding issues are past our demarcation for support but we will always do all we can. 

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New Member

Re: PORT 21 blocked COMCAST BUSINESS -- NOT Xfinity

Hello, @CarlStreet , please click on my name and use the send message option to send a private message.

1. " please click on my name" --   Did so

2.  "use the send message option to send a private message." -- HUH?? There is NO  Send Message OptionThis is what I See -- NO Private Message OptionThis is what I See -- NO Private Message Option

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Official Employee

Re: PORT 21 blocked COMCAST BUSINESS -- NOT Xfinity

Hi there! I'd be glad to ensure that this gets looked into further. Please click on my handle (ComcastRob) and send a private message with your name, the complete service address (including city, state, ZIP, suite number, etc), and the phone or account number, and any pertinent details-- like the ticket number(s)-- so we can help. @CarlStreet

 
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