The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Addressing Concepts and Issues

IP Addressing Concepts and Issues 1 2 3 IP Address Size, Address Space and "Dotted Decimal" Notation

In the introduction of this section, I mentioned that IP addressing is important because it facilitates the primary function of the Internet Protocol—the delivery of datagrams across an internetwork. Understanding this in more detail requires us to examine a few different but essential issues related to how IP and its addresses operate.

The first point that bears making is that there are actually two different functions of the IP address:

• Network Interface Identification: Like a street address, the IP address provides unique identification of the interface between a device and the network. This is required to ensure that the datagram is delivered to the correct recipients.
  
  
• Routing: When the source and destination of an IP datagram are not on the same network, the datagram must be delivered “indirectly” using intermediate systems, a process called routing. The IP address is an essential part of the system used to route datagrams.

You may have noticed a couple of things about this short list. One is that I said the IP address identifies the network interface—not that it identifies the device itself. This distinction is important because it underscores the concept that IP is oriented around connections to a large “virtual network” at layer three, which can span multiple physical networks. Some devices, such as routers, will have more than one network connection: they must, in order to take datagrams from one network and route them onto another. This means they will also have more than one IP address, one per connection.

You might also find it curious that I said the IP address facilitates routing. How can it do that? The answer is that the addressing system is designed with a structure that can be interpreted to allow routers to determine what to do with a datagram based on the values in the address. Numbers related to the IP address, such as the subnet mask when subnetting is used, support this function.

Let’s now look at some other important issues and characteristics associated with IP addresses in general terms.

IP Addressing Concepts and Issues 1 2 3 IP Address Size, Address Space and "Dotted Decimal" Notation