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I have a block of static IP addresses from my ISP (Centurylink). The block is 8 IPs, 5 of which are usable. Lets say

x.x.x.0 - Network Reserved

x.x.x.1 - Assignable

x.x.x.2 - Assignable

x.x.x.3 - Assignable

x.x.x.4 - Assignable

x.x.x.5 - Assignable

x.x.x.6 - Gateway

x.x.x.255 - Broadcast (I could be wrong on this address)

255.255.255.248 - Netmask

Router is Zyxel 2100z. Internet is Fiber 100/50

I have four machines with their networks configured for static IP. Two Centos boxs, an Ubuntu box, and a windows box. They are assigned .1, .2, .4, and .5 respectively.

If I google "what is my ip" on the windows box it returns "x.x.x.6".

If I execute "dig +short myip.opendns.com @resolver1.opendns.com" on any of the linux machines it returns "x.x.x.6".

If I google "what is my ip" on any machine being issued a dhcp lan address (ie. 192.168.0.x) it returns "x.x.x.6".

I expect the last result. However, the other queries should return their respective static IP addresses not the routers address, correct?. Basically, all the traffic going out "appears" to be from x.x.x.6. Otherwise it is working correctly. I can ping all the attached IPs from anywhere. Domain info works. I have a domain name, and sub domain pointed at two of the addresses and those work, and I can ping them. Both by IP and domain name. I'm not sure what issues this can cause, but I looked into it because I'm having weird issues with one of the machines running a webserver.

What could be causing this behavior. I have had static IP blocks in the past, from centurylink, and have not had this issue. I think the modem is configured correctly.

  • If you attempt to ping one of your addresses do you get a result back? Are you 100% CentryLink has actually provided the addresses they claim they have? Honestly it sounds like you have a single address, assigned to your modem, which is what everyone has (not saying that is what your suppose to have) – Ramhound Dec 13 '18 at 5:49
  • "I'm not sure what issues this can cause, but I looked into it because I'm having weird issues with one of the machines running a webserver" – um, could you describe those? – grawity Dec 13 '18 at 6:15
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x.x.x.255 - Broadcast (I could be wrong on this address)

No. Because your netmask is /29 (255.255.255.248) and the first address was a x.x.x.0, the last address in the subnet – and therefore the broadcast address – is x.x.x.7.

I expect the last result. However, the other queries should return their respective static IP addresses not the routers address, correct?. Basically, all the traffic going out "appears" to be from x.x.x.6.

Sounds like your router simply has NAT (masquerading) enabled. Disable it.

(Yes, it's the same kind of NAT as commonly used with private networks. The router probably had it enabled by default.)

Otherwise it is working correctly. I can ping all the attached IPs from anywhere.

Yes, that's expected to work – stateful outbound NAT does not actually block any inbound packets, and the state prevents outgoing replies from being incorrectly translated. (NAT is not a firewall. NAT is not a firewall. NAT is not a firewall.)

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