Chapter Nine: Advanced Installations | IP routes 120 |
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IP routes
This section provides background information on why IP routes are needed and explains how to define them. However, before you can understand routing, you must be familiar with IP addressing and how the DIVA LAN ISDN Modem assigns them.
About IP addresses
IP addresses identify the computers (and other devices) on an IP network. Each address is composed of 32 bits. To make things easier to understand, addresses are written out as four decimal numbers separated by dots. The numbers can range in value from 0 to 255. For example: 192.168.1.1 or 222.182.22.39
Using masks
IP addresses are composed of two parts: the network address and the host address. A host is any computer (or other device) on a network that is capable of transmitting or receiving data. To determine which portion of the address identifies the network and which portion identifies a host, a mask is used.
Masks are specified in bits. A logical bitwise AND operation is performed between the address and the mask to extract the network address. To simplify, consider that each of the four decimal numbers that make up an address is represented by 8 bits. This leads to the following network addresses and their corresponding masks:
Mask | Mask | Source | Extracted |
(# of bits) | (in decimal) | IP address | network address |
8 | 255.0.0.0 | 222.182.22.39 | 222.0.0.0 |
16 | 255.255.0.0 | 222.182.22.39 | 222.182.0.0 |
24 | 255.255.255.0 | 222.182.22.39 | 222.182.22.0 |
32 | 255.255.255.255 | 222.182.22.39 | 222.182.22.39 |
This table also illustrates how the mask is used to extract the network address for a given source address. Note that when the mask is 32 bits, the extracted address is identical to the source address, and refers to a single host computer rather than a network.
Static vs. dynamic addresses
IP addresses can be either dynamic or static. Static addresses are fixed, and must be changed manually. They are used by computers that are permanently connected to the Internet (a web server, for example), and sometimes by computers on a LAN.
Dynamic addresses can change. They are assigned using a special protocol such as DHCP or PPP (IPCP). For example, when connecting to the Internet, the IP address assigned to the DIVA LAN ISDN Modem by your ISP is usually dynamic, assigned by PPP (IPCP). It changes each time the DIVA LAN ISDN Modem connects.
The DIVA LAN ISDN Modem and IP addresses
The DIVA LAN ISDN Modem can have up to three IP addresses assigned to it. One address in used on the internal LAN it creates. The other two are used to communicate externally with remote sites.
Internal LAN addresses
By default, the DIVA LAN ISDN Modem is configured to act as a DHCP server. This means it is responsible for assigning IP addresses to each computer connected to it. To receive an address, each computer must be configured to obtain their IP addresses automatically from a DHCP server (see Chapter Nine).
