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Blocked or restricted ports

Why do we filter ports?
  • Protecting customers - Certain ports are filtered to protect our customers. We can protect against certain common worms and from dangerous services on our customers' computers that could allow intruders access.
  • Protecting upstream bandwidth - Upstream bandwidth to a cable plant is limited. If customers overuse their upstream bandwidth by running high-traffic servers or becoming infected with a worm or virus, it can degrade the service of other customers on that node.
  • Protecting the rest of the Internet - Some filters prevent our customers from attacking other computers on the Internet. In addition to being in our best interests for protecting our bandwidth, it is our responsibility to prevent abuse of our network.
Ports that are filtered
Port Transport Protocol Direction Reason for Filtering
25 TCP SMTP Both* SMTP Relays
80 TCP HTTP Inbound Web servers, worms
135 UDP NetBIOS Both Net Send Spam / Pop-ups, Worms
136-139 UDP, TCP NetBIOS Both Worms, Network Neighbourhood
445 TCP MS-DS / NetBIOS Both Worms, Network Neighbourhood
1433 TCP MS-SQL Inbound Worms, Trojans
1434 UDP MS-SQL Inbound Worms, SQLslammer
1900 UDP MS-DS / NetBIOS Both Worms, Network Neighbourhood

*SMTP is only permitted outbound.

Why each port is filtered
Port Explanation
25 / TCP-SMTP Mail servers use SMTP to exchange email. We block this to protect upstream bandwidth and prevent customers from running open relays. This could be used by others to send spam through our network.
80 / TCP-HTTP Web browsers use HTTP to communicate with web servers. In addition to protecting bandwidth by preventing customers from running high-traffic web servers, we can stop many destructive worms that spread through security holes in web server software.
135, 137 / UDP, 135, 139 / TCP, 445 MS-DC – NetBIOS NetBIOS is a networked file sharing protocol. The Microsoft Windows “Network Neighborhood” runs over NetBIOS. We filter these ports to protect customers from exposing files on their computers, and to block worms which spread through open file shares.
1433 / TCP, 1434 / UDP–MS-SQL Microsoft SQL Server is a database application with a long history of security exploits, and is noted for the propagation of the SQLslammer worm. These ports are filtered to prevent exploitation and propagation of such MS-SQL exploits.
1900 / UDP - UPnP Discovery / SSDP is a service that runs by default on WinXP. It creates an immediately exploitable security vulnerability for any network. Filtering this port proactively prevents XP systems from being remotely compromised by malicious worms or intruders.

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