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I am on the west coast and I am using Sonic fiber internet (sonic.com). The modem provided by the ISP is Pace modem. I am running Ubuntu.

In the modem configuration, I saw that my public IP is 198.xx.xxx.xx, and when I do "ifconfig" from the shell, my interface wlp3s0 has IP address 172.yy.y.yy.

From outside, I can log into my computer from either address (i.e., from both 198.xx.xxx.xx and 172.yy.y.yy).

My modem has been configured to pass ssh onto my computer. If 198.xxx is the modem's NIC, then is 172.yy my computer's NIC? If 172.yy is my computer's NIC, then shouldn't its address start with 192.168.zz.zz? Shouldn't my computer not have a publicly accessible address other than through the modem's NIC (now it seems 172.yy is my computer's public address)?

migrated from serverfault.com Sep 25 '18 at 7:31

This question came from our site for system and network administrators.

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Your assumption that 172.y.y,y is a public ip-address is probably flawed...

Please note that 192.168.0.0/16 is not the only private IP-address space, RFC 1918 also defines the ranges:

  • 172.16.0.0 - 172.31.255.255 (172.16.0.0/12 prefix)
  • 10.0.0.0 – 10.255.255.255 (10.0.0.0/8 prefix)

So probably you have a very conventional NAT modem/router with only a single public IP-address, not two, and private ip-address space "behind" that for your LAN.

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