Network ID Guidelines:

The network ID identifies the TCP/IP hosts that are located on the same physical network. All hosts on the same physical network must be assigned the same network ID to communicate with each other.

Follow these guidelines when assigning a network ID:

The network ID must be unique to the IP internetwork. If you plan on having a direct routed connection to the public Internet, the network ID must be unique to the Internet. If you do not plan on connecting to the public Internet, the local network ID must be unique to your private inter network. 

The network ID cannot begin with the number 127. The number 127 in a class A address is reserved for internal loopback functions.

All bits within the network ID cannot be set to 1. All 1s in the network ID are reserved for use as an IP broadcast address.

All bits within the network ID cannot be set to 0. All 0s in the network ID are used to denote a specific host on the local network and are not routed. 

Network Address

The network address is, strictly speaking, the IP address bitwise-ANDed to the netmask. In English, what this means is that it’s the network portion of your IP address, so if your IP address is 147.120.46.7 and it’s a Class B network, the network address is 147.120.0.0.To get your own network address, just drop the device-specific part of the IP address and set it to zero. A Class C network with an IP address of 201.12.5.23 has a network address of 201.12.5.0. If you’re working with only a loopback address, you don’t need a network mask.

Broadcast Address

The broadcast address is used when a machine wants to send the same packet to all devices on the network. To get your broadcast address, you set the device portion of the IP address to 255. Therefore, if you have the IP address 129.23.123.2, your broadcast address will be 129.23.123.255. Your network address will be 129.23.123.0. If you are configuring only a loopback driver, you needn’t worry about the broadcast address.

 

0 and 255 have special meanings.

0 is reserved for machines that dont know their address. In certain circumstances it is possible for a machine not to know the number of the network it is on, or even its own host address. For example, 0.0.0.23 would be a machine that knew it was host number 23, but didnt know on what network. 

255 is used for "broadcast". A broadcast is a message that you want every system on the network to see. Broadcasts are used in some situations where you dont know who to talk to. For example, suppose you need to look up a host name and get its Internet address. Sometimes you dont know the address of the nearest name server. In that case, you might send the request as a broadcast. There are also cases where a number of systems are interested in information. It is then less expensive to send a single broadcast than to send datagrams individually to each host that is interested in the information. In order to send a broadcast, you use an address that is made by using your network address, with all ones in the part of the address where the host number goes. For example, if you are on network 128.6.4, you would use 128.6.4.255 for broadcasts. How this is actually implemented depends upon the medium. It is not possible to send broadcasts on the Arpanet, or on point to point lines. However it is possible on an Ethernet. If you use an Ethernet address with all its bits on (all ones), every machine on the Ethernet is supposed to look at that datagram. 

Although the official broadcast address for network 128.6.4 is now 128.6.4.255, there are some other addresses that may be treated as broadcasts by certain implementations. For convenience, the standard also allows 255.255.255.255 to be used. This refers to all hosts on the local network. It is often simpler to use 255.255.255.255 instead of finding out the network number for the local network and forming a broadcast address such as 128.6.4.255. In addition, certain older implementations may use 0 instead of 255 to form the broadcast address. Such implementations would use 128.6.4.0 instead of 128.6.4.255 as the broadcast address on network 128.6.4.

 

Finally, certain older implementations may not understand about subnets. Thus they consider the network number to be 128.6. In that case, they will assume a broadcast address of 128.6.255.255 or 128.6.0.0. Until support for broadcasts is implemented properly, it can be a somewhat dangerous feature to use. Because 0 and 255 are used for unknown and broadcast addresses, normal hosts should never be given addresses containing 0 or 255. Addresses should never begin with 0, 127, or any number above 223. Addresses violating these rules are sometimes referred to as "Martians", because of rumors that the Central University of Mars is using network 225.

Special IP Addresses

Any component of an IP address with a value all bits 0 or all bits 1 has a special meaning

all bits 0

stands for ``this: ``this host (IP address with <host number>=0) or ``this network (IP address with <network number>=0) and is only used when the real value is not known. This form is only used in source addresses when the host is trying to determine its IP addresses from a remote server. The host may know include its host number if known, but not its subnet or network number.  

all bits 1

stands for ``all: ``all networks or ``all hosts. For example, 128.2.255.255 (a class B address with a host number of 255.255) means all hosts on network 128.2. These are used in broadcast messages, as described below.

There is another address of special importance: the ``all bits 1 class A network number 127 is reserved for the loopback address. Anything sent to an address with 127 as the value of the high order byte, for example 127.0.0.1, must not be routed via a network but must be routed directly from the IP implementations output driver to its input driver.   

all bits 0

Stands for  this: this host (IP address with <host address>=0) or this network (IP address with network address>=0). When a host wants to communicate over a network, but does not yet know the network IP address, it may send packets with <network address>=0. Other hosts on the network will interpret the address as meaning  this network. Their reply will contain the fully qualified network address, which the sender will record for future use.

all bits 1

Stands for  all: all networks or all hosts. For example, the following means all hosts on network 128.2 (class B address): 128.2.255.255 This is called a directed broadcast address because it contains both a valid <network address> and a broadcast <host address>.

Loopback

The class A network 127.0.0.0 is defined as the loopback network. Addresses from that network are assigned to interfaces that process data inside the local system and never access a physical network (loopback interfaces).

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