Unlike network hubs and regular repeaters that usually broadcast every single packet, LAN switches are able to filter and forward packets by their MAC address (Media Access Control). The switch basically reads the 48-bit MAC address from the network card and is able to filter or stop frames inside the LAN or a certain network domain.

A switch or network bridge will forward frames with addresses that are not in its domain, and will duplicate and broadcast frames to the devices inside its network.

A router can also filter, forward or drop packets based on the MAC addresses, but it can also act based on the Internet Protocol (IP). Routers also act on reducing the collision domain by broadcasting to the LAN only packets that have addresses on that certain network. These network devices are able to route or redirect packets not only by the IP or MAC addresses, but also by the data type (email, graphics, plain text), function or port used (FTP, HTTP, SMTP, POP3) and other variables and functions (acting like a firewall) in order to improve the performance and also reduce the number of collisions and control/limit users.

Ping the loopback address – type ping 127.0.0.1
Successfully pinging the loopback address verifies that TCP/IP is both installed and configured correctly on the local client. If your loopback test fails, then it means IP stack is not answering. If any TCP drivers get corrupted, or if your network adapter is not functioning properly, or if any of the other service is interrupting IP , then lack of response might can occur. Open event viewer, and look for problems reported by setup or by the TCP/IP service.

Ping the local client – type ping
Successfully pinging with the local client's IP address verifies that the client was successfully added to your network. If you cannot successfully ping the local IP address after successfully pinging the loopback address, check that the local client's IP address is a valid IP address, check the routing table, and check network adapter driver.

Ping the default gateway – type ping
Successfully pinging with the default gateway of the local client verifies that you can properly communicate with the local subnet to your local host and your default gateway is also functioning properly. If you cannot successfully ping the default gateway after successfully pinging the local client, check the default gateway.

Ping the IP address of network device located on a remote network – type ping
Successfully pinging with the IP address of the remote host verifies that the local client can communicate with the remote host through a router. if the remote host is located across a high delay link such as satellite link, try using the -w (wait) parameter to specify a longer time out period than the default time out of four seconds.

If you cannot successfully ping the remote host IP address after successfully pinging the default gateway, this indicates that there is no respond from the remote host, or if there is any network hardware problem between the source host and the destination host. To rule out the possibility of a problem in the work hardware, ping to a different remote host on the same subnet where the first remote host is located.

Ping the host name of another host on a remote network – type ping
Successfully pinging with the name of the remote host verifies that ping can resolve the remote host name to an IP address. If you cannot successfully ping the remote host name after successfully pinging the IP address of the remote host, the problem is with host name resolution, but not with network connectivity. When pinging the host name of the target host, ping attempts to resolve the name to an address (first through a DNS server, and next through a WINS server, if one is configured), and then attempts a local broadcast. Check TCP/IP properties to see whether the client has DNS server and WINS server addresses configured, either typed manually or assigned automatically. If DNS and WINS server addresses are configured in TCP/IP properties, and if they appear when you type ipconfig/all, then try pinging with server addresses to ascertain whether they are accessible.

On a network that uses DNS for name resolution, if the name entered is not a Fully Qualified Domain Name (FQDN), the DNS name resolver appends the computer's domain name or name to generate the FQDN. Name resolution might fail if you do not use an FQDN for a remote name. These requests fail because the DNS name resolver appends the local domain suffix to a name that resides elsewhere in the domain hierarchy.

The term of ‘collision domain’ is also used when describing the circumstances in which a single network device sends packets throughout a network segment and forces every other device in that network segment to pay attention to those packets.

CSMA/CD and Collision Domains
A collision domain can also be a group of Ethernet/Fast Ethernet devices in a Local Area Network running on the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) feature and being connected through repeaters, thus competing for network access. Since only one device in the same collision domain can transmit data at a certain point, the other devices in the network simply listen in order to avoid data collisions.

CSMA/CD is a set of rules telling each network devic-e when to transmit and when to stop transmitting data. When someone in the network wants to transmit something, it “listens” to the network at first in order to see if anyone else is using the channel. If no one else is transmitting, the device will go forward with its own transmission.

The usage of CSMA/CD is an efficient way of avoiding network collisions, but it's not foolproof. It's obvious that if two devices follow the exact same procedure at the exact same time, their transmissions will 100% collide, and they will both become unusable. A jam signal will be sent in order to let everyone else know that a collision took place and they should not send any data. The hosts that collided will each start a random timer, and when that ends, each host will begin to listen on the network again.

Of course, the more collisions in a network, the less efficient the network is.

‘Collision domain’ sometimes reffers to a system where a unique identifier is open for multiple interpretations over different layers. The analogy to our ethernet collision domain is very clear if not obvious.

A collision occurs when two or more network devices are trying to transmit packets at the exact same time.

The class of an address specified which of the bits were used to identify the network, the network ID, or which bits were used to identify the host ID, host computer. It also defined the total number of hosts subnets per network. There were five classes of IP addresses: classes A through E.

Classful addressing is no longer in common usage and has now been replaced with classless addressing. Any netmask can now be assigned to any IP address range.

Network and Host ID Fields
The four octets that make up an IP address are conventionally represented by a, b, c, and d respectively. The following table shows how the octets are distributed in classes A, B, and C.

Class IP Address Network ID Host ID
A a.b.c.d a b.c.d
B a.b.c.d a.b c.d
C a.b.c.d a.b.c D

Class A: Class A addresses are specified to networks with large number of total hosts. Class A allows for 126 networks by using the first octet for the network ID. The first bit in this octet, is always set and fixed to zero. And next seven bits in the octet is all set to one, which then complete network ID. The 24 bits in the remaining octets represent the hosts ID, allowing 126 networks and approximately 17 million hosts per network. Class A network number values begin at 1 and end at 127.

Class B: Class B addresses are specified to medium to large sized of networks. Class B allows for 16,384 networks by using the first two octets for the network ID. The two bits in the first octet are always set and fixed to 1 0. The remaining 6 bits, together with the next octet, complete network ID. The 16 bits in the third and fourth octet represent host ID, allowing for approximately 65,000 hosts per network. Class B network number values begin at 128 and end at 191.

Class C: Class C addresses are used in small local area networks (LANs). Class C allows for approximately 2 million networks by using the first three octets for the network ID. In class C address three bits are always set and fixed to 1 1 0. And in the first three octets 21 bits complete the total network ID. The 8 bits of the last octet represent the host ID allowing for 254 hosts per one network. Class C network number values begin at 192 and end at 223.

Class D and E: Classes D and E are not allocated to hosts. Class D addresses are used for multicasting, and class E addresses are not available for general use: they are reserved for future purposes.

This means that if you try to connect to 127.0.0.1, you are immediately looped back to your own machine.

If you telnet, ftp, etc… to 127.0.0.1, you are connected to your own machine.

In other words, 127.0.0.1 is you.

For example, if your system was named “joker”, and you attempted to telnet to 127.0.0.1, you would see:

# telnet 127.0.0.1
Trying 127.0.0.1…
Connected to joker
Escape character is ‘^]’.
Convincing newbie’s to connect to 127.0.0.1 is a frequent joke on the Internet.

Another name for 127.0.0.1 is localhost.

Although 127.0.0.1 is the most commonly utilized address for localhost, any IP address in the 127.*.*.* range should also function in the same manner.