At home, we subscribe to ATT broadband service (fiber optic in fact). Naturally, they have leased and installed a router in out home to act as the gateway to the Internet for all the devices on our private network. This router has a browser based GUI that can be assessed by the router's private IP address.
I was looking around the GUI because I am hoping to fill in some gaps in my knowledge about Networking. I became confused because I am seeing two public IP addresses listed in the 'Broadband Status' tab of the router GUI; fake IPs for this example:
- Broadband IPv4 Address: 87.91.35.144
- Gateway IPv4 Address: 87.91.35.1
What is the difference between these two public IP addresses?
In my current mental model, I assume that the Router is acting as a Network Address Translation (NAT) device and DHCP server; the router has one public IP address allocated to it, one private IP address, and also, in its capacity as a DHCP server, distributes private IP addresses on the same subnet (/24 CIDR) as the private IP it possesses to all the devices in our home. So why are seemingly two public IP addresses displayed on the router GUI? The public IP address that the "World" sees is the one listed on the 'Broadband IPv4 Address' designation: 87.91.35.144 — by "World" I mean the IP address that the Internet tells me it sees when I visit as site like or
My guess would be that the 'Gateway IPv4 Address' is exactly the router/NAT device sitting in my home and servicing my LAN devices while the 'Broadband IPv4 Address' is another device on the ATT network, possibly an aggregator, like an edge router that is collecting a bunch of ATT client traffic from customers all over the ATT network before sending them off to some other Autonomous System (AS) or peer network to reach their destination. That is my guess but I do not have the experience to be sure.
I want to know because I am learning to configure a network firewall and the WAN interface should be configured with the IP address of a public (Internet) facing gateway (router I think). I suppose I could just trial-and-error with only two addresses but I would rather improve my mental model.
11 Answer
In my current mental model, I assume that the Router is acting as a Network Address Translation (NAT) device and DHCP server; the router has one public IP address allocated to it, one private IP address, and also, in its capacity as a DHCP server, distributes private IP addresses on the same subnet (/24 CIDR) as the private IP it possesses to all the devices in our home. So why are seemingly two public IP addresses displayed on the router GUI?
Home routers don't aim for precise terminology. They just want to show a quick summary, with several bits of relevant info in one place.
Here "Broadband" doesn't describe the type or purpose of the IP address – it is not a counterpart to "Gateway". Instead, that's simply your own IP address, the one that is assigned to the router's WAN/broadband interface.
The other address comes not from an interface but from the router's routing table. It's the "gateway" or "nexthop" address from the current default route (i.e. the 0.0.0.0/0 route).
In IP routing, a route's gateway/nexthop has to be a device on the same physical subnet, so the actual router needs to have an IP address that is within your own IP subnet. Here it seems that both you and your upstream gateway are in the 87.91.35.0/24 network (rough guess).
My guess would be that the 'Gateway IPv4 Address' is exactly the router/NAT device sitting in my home and servicing my LAN devices while the 'Broadband IPv4 Address' is another device on the ATT network, possibly an aggregator, like an edge router that is collecting a bunch of ATT client traffic from customers all over the ATT network before sending them off to some other Autonomous System (AS) or peer network to reach their destination. That is my guess but I do not have the experience to be sure.
No – it's actually the opposite. "Broadband IPv4 address" is your router's own address, while "Gateway IPv4 address" is... another router that your router sends traffic to. In other words, the latter describes the "default route" aka "default gateway" that is configured in the router itself.
Here "gateway" does not mean "customer's gateway to the internet", it also has the older meaning of "an IP router".
(Your computer's "default route" points at your router; your router's "default route" points at some ISP router just on the other end of the physical connection; and that router probably still has a "default route" pointing somewhere next, because AT&T's network is large, so most likely the AS border routers that have a full BGP routing table are still 1–3 hops further.)
