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Does a cable modem ever block inbound traffic?

Posted on 2011-05-03
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I have a D-Link DIR-655 wireless router connected to a Comcast (SMC) cable modem (SMC: S/W Rev: 1.4.0.46-CCR, SMCD3G-CCR).  

I'm going to open some inbound ports and configure port forwarding on the router.  Do I need to worry about the cable modem blocking inbound traffic?

I don't think I need to worry (but I've been wrong before).  After all, it's just a layer 2 network bridge, but I think I remember that some cable modems do have TCP / UDP port filtering functionality.
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Question by:jdana
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Mike Thomas earned 144 total points
ID: 35513349
The cable modem itself will not block inbound traffick unless it is a router type cable modem device with that added functionality, but it won't be anything you cannot configure.
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by:emilgas
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ID: 35513567
You are right, it's just a Layer 2 bridge. It won't block anything
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by:akahan
akahan earned 144 total points
ID: 35513762
Even though the cable modem itself would not block traffic, your ISP (especially if your ISP is Comcast...) might be shaping or blocking traffic on some ports.  You might want to try this free utility to find out if that's the case:  http://broadband.mpi-sws.org/transparency/bttest-mlab.php

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by:jkeegan123
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The ISP will definitely block traffic for a basic subscriber (read NON-BUSINESS account).  I believe that this is done at the modem level by most greater NY tri-state areas ISPs like Cablevision.  Mostly they will block common server ports, like port 21 (FTP), port 25 (SMTP), and port 80 (inbound web traffic).
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by:houssam_ballout
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you may need to check your computer (or router) if its blocking some ports. Typically, modem won't block traffic.
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by:jkeegan123
jkeegan123 earned 288 total points
ID: 35513898
Cablevision (www.cablevision.com) DEFINITELY blocks port 25, 80 at the modem.  Your mileage may vary.
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by:emilgas
emilgas earned 284 total points
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In that case I should be very lucky, I have Time Warner Cable and nothing is being blocked.
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Author Closing Comment

by:jdana
ID: 35805650
Thanks for all the good feedback.
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