t. As the Gi interface interacts with external networks, security is also an important issue. Firewalls are used between the GGSNs and external networks. A firewall protects both the subscribers and the QoS of the network (by not letting it become overloaded). Connection between the GGSN and an external network is done through virtual routers (also known as access points). A gateway tunnelling protocol (GTP) in the GPRS network is responsible for routing of the information either way. Another important aspect that is taken care of by the core planning engineers is IP addressing of the network elements. Some fundamentals of IP addressing and routing are given below. Basics of IP addressing and Routing IP Addressing Data exchange between two network elements is a three-step process: addressing, routing and multiplexing. Every network element in the Internet should have a unique IP address, making it possible for data to reach the right host. Routers target the data to the correct network, while multiplexing makes it possible for the data to reach the correct software within the host. There are three classes of IP address, A, B and C. The number of networks in each class can be computed as: class A: 128 class B: 64 X 256 = 16 128 class C: 32 X (256)2 = GPRS NETWORK PLANNING The number of addresses in each class can computed as: class A: (256)³ = 16777216 class B: (256)2 = 65 536 class C: 256. Thus, class A has the least number of networks and the largest number of addresses, while class C is has the largest number of networks and the least number of addresses. How can one identify to which class an address belongs? The addresses are of 32-bit length. If the address starts with bit 0, then it is a class A address. Class B addresses start with bits 1,0; while class C addresses start with 1,1,0. Class A: The first bit 0 acts as class identifier; the next seven bits as network identifier; the final 24 bits as host identifier. Class B: The first two bits 0,1 act as class identifier; the next 14 bits as network identifier