Designing TCP/IP Based Networks.

Some nodes may use multiple IP addresses.

Networks come in classes A,B,C,D,or E, though classes D and E addresses are reserved for special purposes.


When troubleshooting rather than examining the whole network, a network administrator should only have to see that the faulty transmissions are all associated with addresses in a certain area to know that they should zero in on that subnet.

The 1 bits in a subnet mask indicates that corresponding bits in an IPv4 address contain network information. The 0 bits in a subnet mask indicate that corresponding bits in an IP address contain host information.


Class A = Default Subnet Mask = 11111111 00000000 00000000 00000000 = Number of bits used for network information = 8 =

Class B = 11111111 11111111 00000000 00000000 = 16 =

Class C = 11111111 11111111 11111111 00000000 = 24 =


Notice that the address's fourth octet could have been composed of any combination of 1s and 0s, and the network ID's fourth octet would still be all 0s.

When using classful IPv4 addressing, a network ID always ends with an octet of 0, and may have additional, preceding octets equal to 0: however, when subnetting is applied and a default subnet mask is no longer used. A network ID may have other decimal values in it's last octets.

Another reserved IP address is the broadcast address for a network or segment. In broadcast address, the octets that represent the host information are set to equal all 1s, or in decimal notation, 255.

Because the octets equal to 0 and 255 are reserved, only numbers 1 through 254 can be used for host information in an IPv4 address.

Subnetting breaks the rules of classful IPv4 addressing.

By, 1993, the Internet was growing exponentially, and the demand for IP addresses was growing with it. The IETF (Internet Engineering Task Force) recognized that additional measures were necessary to increase the availability and flexibility of IP addresses. In response to this need, the IETF devised CIDR (Classless Interdomain Routing), which is sometimes called classless routing or supernetting. CIDR is not exclusive of subnetting: it merely provides additional ways of arranging network and host information in an IP addess. In CIDR, conventional network class distinctions do not exist.

In CIDR terminology, the forward slash, plus the number of bits used for extended network prefix - for example, /22 - is known as a CIDR block.

Every device on a TCP/IP-based network has a default gateway - that is, the gateway that first interprets its outbound requests to other subnets, and then interprets its inbound requests from other subnets.

A gateway is analogous to your local post office, which gathers your outbound mail and decides where to forward it.

The gateways that make up the Internet backbone are called core gateways

A public network is one that any user may access with little or no restrictions. A private network is a network whose access is restricted to only clients or machines with proper credentials. Virtually all business LANs and WANs are private networks.

When the client's transmission reaches the default gateway, the gateway opens the IP data gram and replaces the client's private IP address with an Internet-recognized IP address. This process is known as NAT (Network Address Translation).

Any Internet-vallid IP address might be assigned to any client's outgoing transmission. This technique is known as DNAT (Dynamic Network Address Translation).