Every device that connects to Comcast xfinitywifi gets 192.168.1.10, how?

Question

There is an xfinitiwifi hotspot near me.

Comcast users ("slaves"?) can connect to xfinitywifi by entering their Comcast email and password on the browser redirect. Then it will record your MAC address and let you access later without the popup.

But the weird thing is, every time you login you get the IP address 192.168.1.10. Even if you login with multiple devices. (Also documented here: http://forums.comcast.com/t5/Home-Networking-Router-WiFi/What-Prevents-Xfinity-WiFi-Hotspot-Spoofing/td-p/2113294 )

How is it possible to have an access point reuse the same IP address for each client?

Answer 1

I suppose the AP could effectively put each associated client in its own VLAN, and not do intra-BSS relay. Then, somewhere on the network, they could run a separate NAT instance for each VLAN.

The only problem left to solve would be broadcasts from the network. If the router on one of the VLANs tries to send an ARP "who-has 192.168.1.10?" broadcast, all the clients would reply. But their replies would each go to a different VLAN, and all the ARP tables would be updated, making other VLANs' routers less likely to need to send ARP broadcasts as soon.

Alternatively, the network could employ other tricks to keep the ARP table up to date without requiring broadcast ARP requests from the router.

Honestly though, even though there are ways to make the network extra clever to make this work, I don't see why they'd go to these lengths.

Answer 2

A combination of vlans, tagging and mac/ip binding is what makes it work. I mean its only a class c ip with a prefix of /24.

The maximum number of VLANs on a given network is 4,094. Comcast generally issues Class B ip's 10.0.0.1/11 (255.224.0.0). Regardless each new client is assigned the necessary information via DHCP. However, the client was not only assigned an IP but also a virtual lan number. This isolates the vlan from everything except its gateway. The host can now assign the same lan segment and prefix (192.168.1.0/24) to each client as each vlan is autonomous of the other, except for its gateway. Finally, vlan tagging and arp binding provides routing for those clients who share the same class c ip address.