Beginning networking- IP addressing

[Czar]

Cisco class notes- Early on

Topic: Ip addressing-Subnets-Classes

From: Cisco Classes [yes.. i have the cert.. if you want any topics just ask..]

Reason: In order to understand what the rest is you must know the basics, and other than hardware this is fairly basic.

One of the most important aspects of communications on an internetwork is the IP addressing scheme.

IP addressing is the method used to identify hosts and network devices. As the Internet grew over time and the number of hosts connected to it increased, IP addressing schemes had to adapt to cope with the growth.

While IP addressing schemes have had to adapt, the basic IP address structure for IPv4 remains the same. To send and receive messages on an IP network, every network host must be assigned a unique 32-bit IP address. Because large binary numbers are difficult for people to read and understand, IP addresses are usually displayed in dotted-decimal notation. In dotted-decimal notation, each of the four octets is converted to a decimal number separated by a decimal point. For example, the IP address:

11000000.10101000.00000001.01101010

is represented as 192.168.1.106 in dotted-decimal notation.

IP addresses are hierarchical. A hierarchy is like a family tree with parents at the top and children connected to them below. For a network, this means that part of the 32-bit number identifies the network (parent), while the rest of the bits identify the host (child). In the early days of the Internet, there were so few organizations needing to connect to the Internet, that networks were assigned by only the first 8 bits (first octet) of the IP address. This left the remaining 24 bits to be used for local host addresses.

The 8-bit network designation made sense at first, because originally people thought that the Internet would be made up of a few very large universities, governments, and military organizations. Using only 8 bits for the network number enabled the creation of 256 separate networks, each containing over 16 million hosts. It soon became apparent that more organizations, and eventually individuals, were connecting to the Internet to do research and to communicate with others. More networks were required, and a way to assign more network numbers had to be created.

To create more possible network designations, the 32-bit address space was organized into five classes. Three of these classes, A, B, and C, provide addresses that can be assigned to individual hosts or networks. The other two classes, D and E, are reserved for multicast and experimental use.

Until this change, routers examined only the first 8-bits of an IP address for the network ID. Class B networks, however, use the first 16 bits to identify the network. Class C networks use the first 24 bits to identify the network. With this addition, routers needed to be programmed to look beyond the first 8 bits to identify class B and C networks.

It was decided to divide the networks in a manner that would make it easy for routers and hosts to determine the correct number of network ID bits. The class of a network is indicated by the values of the first few bits of the IP address, called the high-order bits. If the first bit is 0, the network is a Class A, and the first octet represents the network ID. When the first bit is 1, the router examines the second bit. If that bit is 0, the network is a Class B, and the router uses the first 16 bits for the network ID. If the first three bits are 110, it indicates a Class C address. Class C addresses use the first 24 bits, or three octets, to designate the network. Dividing the original 8-bit network into smaller network classes increased the number of available network designations from 256 to over two million.

In addition to creating separate classes, the Internet Engineering Task Force (IETF) decided to reserve some of the Internet address space for use by private networks. Private networks have no connection to public networks. Private network addresses are not to be routed across the Internet. This allows multiple networks in various locations to use the same private addressing scheme without creating addressing conflicts.

The use of private address space reduced the number of unique registered IP addresses that were assigned to organizations.

A single Class A address, 10.0.0.0, was reserved for private use. In addition, address space in classes B and C was also set aside for private networks.

Most networks today use a private address structure. Most consumer networking devices, by default, give out private addresses through DHCP. Only the devices that connect directly to the Internet are assigned registered Internet routable addresses.

Networks continued to grow and connect to the Internet throughout the 1980s and into the 1990s, with many organizations adding hundreds, and even thousands, of hosts to their network. An organization with thousands of hosts should have been well served by a Class B network, however, there were some problems.

First, organizations with thousands of hosts rarely had them all in one place. Some organizations wanted to separate individual departments from each other for security or management purposes. Second, a primary type of packet forwarded on a network is the broadcast packet. Broadcast packets are forwarded to all hosts within a single logical network. With thousands of hosts on a single network sending broadcast traffic, and limited bandwidth available, network performance significantly decreased as more hosts were added.

To solve these problems, the organizations leading the development of the Internet chose to partition their networks into mini-networks, or subnetworks, using a process called subnetting. How can a single IP network get split into multiple networks so that each subnet is treated as a separate network?

RFC 917, Internet Subnets, defines the subnet mask as the method routers use to isolate the network portion from an IP address. When a router receives a packet, it uses the destination IP address in the packet and the subnet masks associated with the routes in its routing table to determine the appropriate path on which to forward the packet.

The router reads the subnet mask from left to right, bit by bit. If a bit in the subnet mask is set to 1, it indicates that the value in that position is part of the network ID. A 0 in the subnet mask indicates that the value in that position is part of the host ID.

When a network is partitioned, the router must use a modified or custom subnet mask to distinguish the subnets from each other.

A default subnet mask and a custom subnet mask differ from each other in that the default subnet masks only change on octet boundaries. For instance, the default subnet mask for a Class A network is 255.0.0.0. Custom subnet masks take bits from the host ID portion of the IP address and add them to the default subnet mask.

To create a custom subnet mask, the first question to answer is how many bits to take from the host ID to add to the subnet mask? The number of bits to borrow to meet a specific number of subnets can be determined by the math equation: 2^n, where n equals the number of bits borrowed.

If three subnets are required, there must be enough subnet bits to allow for three unique subnet addresses.

For example, if starting with a Class C address, such as 192.168.1.0, there are only eight host bits to borrow from. Each bit can only be a 1 or a 0. To allow for three subnets, at least two of the eight bits must be borrowed. This creates four subnets total:

00 - 1st subnet

01 - 2nd subnet

10 - 3rd subnet

11 - 4th subnet

In the above example, two bits were borrowed, 2^2 = 4 or 2 x 2 = 4, so four subnets were created. If between five and eight subnets were needed, then three bits would be required (2^3 = 8 or 2 x 2 x 2).

The number of bits selected for the subnet ID affects both the number of possible subnets and the number of hosts that can be in each subnet.

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In the original IP address hierarchy, there are two levels: a network and a host. In a classful addressing scheme, the first three leading bit values are used to determine that an IP address is either a Class A, B, or C. When an address is identified by class, the number of bits that make up the network ID and the number of bits that make up the host ID are known. The default subnet masks for the network classes are:

Class A 255.0.0.0

Class B 255.255.0.0

Class C 255.255.255.0

Subdividing a classful network adds a level to the network hierarchy. Now there are three levels: a network, a subnetwork, and a host. How can the subnet mask be modified to indicate the new hierarchical level?

A single Class A, B, or C network address space can be divided into multiple subnetworks by using bits from the host address space to designate the subnet ID. As an example, an organization using a Class C address space has two offices in different buildings. To make the network easier to manage, the network administrators want each location to have a logically separate network. Taking two bits from the host address increases the subnet mask length from the default 24 bits to 26 bits, or 255.255.255.192.

When bits are borrowed from the host portion of the address to identify the subnet, fewer bits are available for individual hosts. If two bits are used for the subnet ID, only six bits are left in the host portion of the address.

[Oglog40]

too much :S

[Shottingham]

lol u can actually go on utube and type in "IP Addressing" or "Subnetting" gives u better tutorials and videos! thts where i learned it.
this is wayyyy too much text.