My ISP terminates their fiber connection with a Ceina router, which then goes to a Cisco router which hands off my internet connection. I only have documentation on the Cisco (2nd device in line) config, not the Ceina (1st device, fiber termination). I noticed on the Cisco config the eth port going to the Ceina is an IP/subnet in the public range, but isn't my static IP block from my ISP. The eth port going to my network from the Cisco router is indeed that block however.

I understand with proper routing this can work. I'm just curious what that 2nd unknown public-space block of IP's is about? Does my ISP forward traffic to that subnet? Doesn't the fact it overlays public IP space make an issue? Wouldn't the Ceina just use up one of my IPs in my block? Anyone seen this before?

1 Answer 1


Many (most?) ISPs use a separate address block for their PE-CE links. It allows them to manage their devices more easily, and it doesn't use up addresses from the blocks they assign to customers (like you). The address block they use is assigned to them, so there's no issue with addressing.

  • After thinking about it, that was my assumption. Thank you for confirmation with your experience.
    – user10411
    Jan 5, 2015 at 21:18
  • I've seen a few that use unnumbered interfaces or private IPs on the WAN side, but most do use public /30s.
    – pooter03
    Jan 6, 2015 at 22:11

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