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I'm trying to block skype at the router and when I add an address such as it says "Invalid IP address:!". I'm using a Cisco DPC3825. Any ideas how to block the IP address?

@Nixda still no good screen shot or router rejecting ip address

Here's a screen shot of under basic rules. I can't understand how to add entries.

access/restrictions/basic rule

I tried adding but I'm not sure if it got added correctly.

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well, dot 0 at the end is wrong wrong. 'cos .0 is for a subnet(for when specifying a range of IPs) not for an individual machine. Try checking the IP of the machine you want to block again. – barlop Sep 29 '13 at 11:40
@barlop I'm following these directions – Celeritas Sep 29 '13 at 11:51
The example address is invalid... – Ramhound Sep 29 '13 at 13:28
@Ramhound No it is a valid IP address. Just to represent a subnet though, one for a single device. As I said. – barlop Sep 29 '13 at 14:52
@Celeritas That webpage says it's for networks (subnets). See the forward slash after the IP address. That means hold the first 24 bits, and vary the last 8. So it's the whole range How to vary - I don't know about that router, you could add a cisco tag to your question and title. – barlop Sep 29 '13 at 14:57
up vote 4 down vote accepted

Some basics you need to know first

  1. isn't a valid IP because an IP with an ending zero is the network identifier

  2. A following backslash with a number is an alternative notation for a subnet mask   <- network identifier  <- subnet mask 

    This subnet mask is in decimal writing. If we convert both numbers to binaries, you will see that there are 8 zeros and 24 ones. That's where the /24 came from

    0110 1111.1101 1101.0100 1010.0000 0000  <- network identifier
    1111 1111.1111 1111.1111 1111.0000 0000  <- subnet mask
    |______ network prefix _____| |_hosts_|
  3. The zeros define (mask) the valid first and last IP for hosts of that network

    0110 1111.1101 1101.0100 1010.0000 0001  <- first valid IP address for hosts
    0110 1111.1101 1101.0100 1010.1111 1110  <- last valid IP address for hosts
  4. The 1111 1111 (=255) is reserved for the broadcast address

    0110 1111.1101 1101.0100 1010.1111 1111  <- broadcast address

Back to your router problem

A look at the Cisco DPC3825 manual (site 60) reveals that your router wants the start and end IP to block a whole range of IP addresses. For the start/end address would be and

enter image description here


You cannot enter these skype IPs under "Access Restrictions > IP Address Filtering". This subpage is only for outbound traffic respectively IPs from local hosts which are connected to your router. Usually they begin with For example, to block your child's laptop you would enter (start:) - (end:) The cisco router checks if an entered IP is within the range of and since it's not he states "invalid IP".

You want to block inbound traffic for specific IPs. As far as I can tell, this seems impossible for your Cisco DPC3825 (which runs a Linksys software btw). Your best chance is under "Access > Basic Rules" since your Skype block guide says

Currently there are 16 DNS entries from to

Try to enter URLs instead of IPs

You may reach the maximum of blocking rules. If this happens, try if wildcards (*, ?) are allowed: dsn*

enter image description here

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I tried but it still says invalid IP address (see update) – Celeritas Sep 30 '13 at 1:02
I see the problem. Read my edit :) – nixda Sep 30 '13 at 7:50
@Celeritas Any updates with your issue? Have you solved it? – nixda Nov 6 '13 at 16:40
Actually I kind of gave up but I'll let you know if I come back to it. – Celeritas Nov 6 '13 at 18:12
@Celeritas Sad to hear :-/ – nixda Nov 6 '13 at 18:13 is a network, not a site. If you you want, you can block all sites belonging to that network by specifying, or alternatively you may block only one specific member of the network, say

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