Subnetwork Performance

You have understood that subnetting does not allow IP addresses to be lost in cyberspace, but there is another fact that needs to be discussed which is very important: performance.

In any network, a host can communicate only with other hosts which have the same network address. Two hosts on different networks can communicate only through a router. A router is a network device which facilitates communication between networks. This machine can be a computer configured to act as a router, but in most cases it's a dedicated device. Cisco is the world best known router manufacturer, their routers, like any other dedicated routers are computers controlled by a network operation system which run routing protocols and switch packets between their interfaces.

I do not want to want to enter into routing details here because I will talk about routers latter, what you need to remember is that a router is a link between two different networks and it's the only networking device able to switch packets at the Layer 3 level.
I have told you earlier that two host IP addresses in the hosts' space are used as network and broadcast addresses. The network address is used by routers in the communication process to select destination networks, while the broadcast address is used to communicate with all hosts in a network in the same time. A common mistake is that many users make assume that *.0 is the network address and *.255 is the broadcast address for any network. Although these addresses are always network and broadcast addresses, no matter what subnet mask is used, these are not the only one. Other addresses are always created through subnetting.

One of the main reasons why we use subnetting is to reduce the size of the broadcast domain. The broadcast domain size is the number of all hosts in a network. Broadcasts are used in conjunction with many network services, being a negative factor in the network performance. In a network a specific host deals only with packages addressed to its IP address, but every host on the network also considers the broadcast packet as it would have been addressed to it. In a large network with many hosts broadcast packages can eat almost all available bandwidth. That's why we have to reduce the number of hosts that listed to these broadcasts because in most cases we can not influence in a significant degree the number of broadcasts.

One important IP address you should remember is the loopback address: 127.0.0.1. This IP address is used in troubleshooting process and it's referred like an "echo address". Every package send to this address loops back to the destination host. If you can not reach other hosts in a network and want to troubleshoot your connection you can try to ping 127.0.0.0. If your network card is properly installed you will get a reply.