I recently found out the ISP provides a /28 public IP subnet to a business. I'm in the process of learning more about networking and find it interesting that 16 addresses are available through that one connection. That line from the WAN terminates at a firewall which then acts as a router or NAT device. I'm not talking about routers at home creating a private subnet for a WAN connection to the house. What are the general concepts involved here?

  • 2
    A single link can route any number of addresses on each side - where exactly is your problem?
    – Zac67
    Commented Jul 8 at 11:48
  • It is either a layer-3 routed link or a layer-2 trunk link.
    – Ron Maupin
    Commented Jul 8 at 13:29
  • Also, understand that IP, itself, has no concept of public or private addresses. To IP, they are all IP addresses. It is an artificial distinction that ISP will not route some address ranges that are designated as private ranges, but it is not built into IP that way.
    – Ron Maupin
    Commented Jul 8 at 14:07

1 Answer 1


Very often when your ISP assigns an IP subnet to you, the CPE router uses one of those addresses to interface with your network on the handover port. There are two more addresses that are unusable, the subnet prefix itself (all host bits zero) and the broadcast address (all host bits one).

All other addresses can be used directly on the handover port, either through a switch by different devices, or by a firewall as an address pool.

It's also possible to assign a single uplink address to your router (like on a consumer link), and then just route the assigned subnet through that link. That way you can use it any way you like in your network/DMZ.

Both scenarios are largely the same, the second one just leaves the CPE job to your router.

  • Thank you for the response. So the 14 addresses are staticly assigned to a single NIC?
    – KSK
    Commented Jul 9 at 1:44
  • 1
    That is one possibility (or by a firewall as an address pool).
    – Zac67
    Commented Jul 9 at 5:29

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