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Chapter (3) Networking Media and Data Link Layers Protocols

4.4.2 Static Assignment of an IP Address

When IP addresses are assigned statically, each device must be configured with an IP
address. Each operating system has its own way of configuring TCP/IP. This method requires
records of the address assignments to be kept, because problems can occur in a network if
duplicate IP addresses are used. Some operating systems, such as Windows 95 and Windows
NT, send an ARP request to check for a duplicate IP address when they attempt to initialize
TCP/IP. If a duplicate is discovered, the operating system does not initialize TCP/IP and generates
an error message. Not all operating systems identify duplicate IP addresses.

This again emphasizes the need for good record-keeping. The main reason that a device
would be assigned a static IP address is if the device needs to be referenced by other devices. A
good example is a web server. If a web server got a new IP address each time it started up, it
would be difficult to find the web server.

As an example of this address changing, if a city were to constantly change street names
and building addresses, maps would no longer help you locate a particular building. If an address
changes, it is no longer easy to return to the location. If a building is difficult to get to, people
will stop trying to locate it. Certain types of devices need to maintain a static IP address. Web
servers, network printers, application servers, and routers are good examples of devices that
require permanent IP addresses. https://www.youtube.com/watch?v=OqsXzkXfwRw

4.5 Address Resolution Protocol

For devices to communicate, the sending device needs the destination device’s IP
address and MAC address. When a device tries to communicate with a device whose IP
addresses it knows, it must determine the MAC addresses.
The TCP/IP suite has a protocol called Address Resolution Protocol (ARP) that can
automatically obtain the MAC address. ARP lets a computer find the MAC address of the
computer that is associated with an IP address, as shown in Figure 4-25. Some devices keep ARP
tables, which contain the MAC addresses and IP addresses of other devices that are connected
to the same LAN.
ARP tables map IP addresses to the corresponding MAC addresses. ARP tables are
sections of RAM memory that are maintained automatically on each device, as shown in Table
4-6 and Table 4-7.
It is rare that you must manually make an ARP table entry. Each computer on a network
maintains its own ARP table.

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