How does a GSM network route internet packets? Does GSM use NAT based PAT like a physical Router? For example, user A visits website #1 and user B visits website #2. how data of website #1 are given to user A instead of B or both of them?
Routing – How GSM network routes Internet packets
gsmmobilenat;routing
Related Solutions
IPv4 or IPv6 doesn't matter, except for the availability of addresses. These are examples using IPv4.
Topology A is what most people do. Topology B is what Mike Pennington said. Topology C is what you can do if you get a few addresses from your ISP.
NAT
If you are using static (one-to-one) NAT, the router will assign the 11.2.10.172 public IP to the first PC ( for example 192.168.1.101 ) trying to reach google.com. In this case, the two PC will not be able to communicate with google.com at once, because the only available public IP is already distributed.
The NAT table in the router:
11.2.10.172 -> 192.168.1.101
PAT
In your case PAT ( NAT overloading ) is the solution.
With PAT, multiple addresses can be mapped to one private IP. When a device initiates a TCP/IP session, it generates a TCP or UDP source port number to uniquely identify the session. When the router receives this packet it uses that source port number to uniquely identify the translation.
Example
PC1 (192.168.1.101) makes an HTTP request to google.com (64.233.161.1) with a random source port number (1444). PC1 will send a packet with DA: 64.233.161.1:80 | SA: 192.168.1.101:1444. When the router receives this packet it inserts 11.2.10.172:1444 -> 192.168.1.101:1444 to the NAT table then changes the L3 addressing of the packet to DA: 64.233.161.1:80 | SA: 11.2.10.172:1444 and forwards it to google.com.
Google responds with DA: 11.2.10.172:1444 | SA: 64.233.161.1:80. The router receives this packet and translates it to DA: 192.168.1.101:1444 | SA: 64.233.161.1:80 then forwards it to PC1.
If PC2 (192.168.1.102) sends a packet with the same source port number as PC1 did , the router simply increases the port number by 1. In that case the NAT table would look like this
11.2.10.172:1444 -> 192.168.1.101:1444
11.2.10.172:1445 -> 192.168.1.102:1444
I hope it helps a bit.
UPDATE
As @CraigConstantine noticed, 10.2.10.172 is still in the private address space so I have changed it to 11.2.10.172.
Best Answer
GSM data networks (GPRS, LTE etc) allocate IP addresses to end devices (Mobile Phones) just like any other network.
As to whether NAT/PAT is used, this is entirely carrier dependent - based on a factor of how big their subscriber base is and how many public IPv4 addresses they have available.
In Australia, the incumbent telco Telstra uses a CGN (carrier-grade NAT) to support the massive numbers of 3G/LTE subscribers currently using it's network. All devices using their default APN are allocated an address from 10.0.0.0/8, and then NATted to the Internet. Public IPs are available, but only on premium services.
In markets where IPv6 is more established (eg: much of Asia), end devices are more likely to be provided with a public v6 address, requiring CGN only where the content being viewed is v4 only.