My static ips are inaccessible only from CenturyLink users across all ports

Altimus Prime asked:

The Problem:

How do you diagnose an ISP specific connectivity problem?

I host a few sites on local machine. I pay for a business class account with Centurylink and have a lot of 5 static ips. The sites are accessible from anywhere in the world except other Centurylink users. If my customers didn’t live in the sticks like me, I wouldn’t have ever known. Cox users, Verizon users, Cricket, At&t, Comcast, and a huge number of US and internationally based internet service providers can access my sites and ip addresses, EXCEPT other people using Centurylink.

What I’ve done:

While on location at another person’s home with Centurylink internet, I attempted to access each ip address on ports 80 and 443. I attempted to ssh into the server at an arbitrary port that I’ve selected for that service. I ran tracert on both the domains and the ip addresses and got the following:

1   2ms 1ms 1ms modem.Home []
2   30ms    30ms    25ms [x.x.123.45]
3   29ms    30ms    33ms [x.x.122.34]
4   29ms    30ms    33ms    x.x.x.18
5   200ms   79ms    59ms [x.x.45.67]
6   *   *   *   Request timed out.
7   *   *   *   Request timed out.
8   *   *   *   Request timed out.
9   *   *   *   Request timed out.
10  *   *   *   Request timed out. 

Coincidentally x.x.45.67 is the ip address I get when I Google What’s my ipv4 into Google. If this were working the static ip address would be the next result, so I would think that the request is being blocked at the router.

Using the originating public ip address I searched the access logs and the firewall logs on the router. There are no records of the remote ip address. I am able to find records when packets are dropped according to the rules I’ve set for OTHER ip addresses.

Just in case I checked the server logs and found no record of the remote address.

Where else could I check to diagnose this problem? I would have asked on SO Network Engineering, but they restrict their questions to only enterprise level issues and solutions.

I’ve checked blacklists, but my ip addresses don’t come up. I’ve checked the router firewall, but when something there is blocked by one of my rules, it gets logged and I can find the record.

My worst case solution

Before someone suggests it, my worst case scenario is to reverse proxy the traffic through some remote servers I also administer.


I set up the reverse proxy last night in less time than the average hold time at Centurylink. I’m loathe to call them because they took a month just to provision my static IP addresses. It was like no one there understood why a business needs a static IP address, or even what that is.


Michael Hampton’s comment about the netmask being incorrect sounds like he’s exactly right. All the clients who can’t access my address share at least the the first 8 bits of the ip address. The netmask was when it should have been I made the change, and restarted the network adapter. Checked if an offending ip could access the ip but still couldn’t. Opened the ifcfg file for the network adapter and the netmask was again. I’ll have to find out why it’s getting reset, but again, Michael Hampton’s comment is most likely the right answer to this problem.

My answer:

It’s most likely your netmask is incorrect. The netmask determines which IP addresses are considered to be on the same layer 2 network segment, and if it is incorrect, you will lose connectivity to addresses within that mask, as your device will think they are on the local layer 2 network segment, and will not attempt to route those packets anywhere.

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