COMPUTER NET WORKING 1

IP ADRESSS

The netmask is used to identify which portion of the IP address represents the network address and which portion represents the machine address^*.

Consider the class A network 10.0.0.0 to 10.255.255.255 (this is the designated "private" class A block i.e. packets destined for these addresses will not be routed). In binary the addresses are: 00001010.00000000.00000000.00000000 through to 00001010.11111111.11111111.11111111. The netmask is generated by assigning a 1 for each bit in these addresses that does not change IE:

00001010.00000000.00000000.00000000

00001010.11111111.11111111.11111111

-----------------------------------

11111111.00000000.00000000.00000000

Which converts to 255.0.0.0, the classic class A netmask.

^* Actually, to get the network address given an IP address, you just do a logical AND. For example, for the netmask 255.0.0.0 and the IP address 10.0.0.1:

00001010.00000000.00000000.00000001

11111111.00000000.00000000.00000000

-----------------------------------

00001010.00000000.00000000.00000000

And 00001010.00000000.00000000.00000000 translates to 10.0.0.0 which is indeed the network address.

Note that you usually don't need a class A network for a home network (do you need more than 255 addresses?) and can thus use 255.255.255.0 as netmask and/or use a class C network (e.g. 192.168.1.0).

Mask comparison: subnet versus wildcard

By stretch | Thursday, September 11, 2008 at 12:21 a.m. UTC

Many newbie network admins express difficulty with the subnet-versus-wildcard masking paradigm. It is a commonly held belief that a wildcard mask is simply the inverse of a subnet mask, and this is often the case, but not necessarily. Here's a brief rundown explaining the purpose of both mask types.

Subnet Masks

Subnet masking is an unfortunate product of the conversion from classful to classless IP routing that took place decades ago. When IP addresses were assigned a class (A through E) based on their first few bits, the class determined the scope of the network: /8 for class A, /16 for class B, and so forth. Classless Interdomain Routing (CIDR) was developed to provide a much more flexible, albeit more complex, address scheme involving the variable-length subnet masks we use today.

Subnet masks serve only to express a length of bits, matching the network portion of an IPv4 address from left to right. A subnet having a mask of 255.255.240.0 is the same as having a "length" of 20, or /20. In fact, there are only 33 possible IPv4 subnet masks, from 0.0.0.0 to 255.255.255.255, or from /0 to /32. So why go through the pain of dotted-decimal notation for such a simple array of values? Binary-to-decimal subnet mask calculation is taught in introductory networking classes simply to serve as a crucible to weed out the weaker network admins in training.*

Wildcard Masks

Wildcard masks are much more flexible than subnet masks, as there is no requirement for contiguity. In the Cisco world, the bits are opposite from subnet masks, so that a 0 matches and a 1 does not. It is common practice for access list wildcard masks to appear as the inverse to the subnet mask of the network being formed; for example, 0.0.0.255 matches any value for the last octet of an IP address in a 255.255.255.0 subnet. But this isn't strictly necessary. One can achieve all sorts of strange matches with a wildcard mask; some examples are given here in IOS ACL syntax:

Match all 192.168.x.1 addresses:

permit 192.168.0.1 0.0.255.0

Match only even 192.168.x.0/24 subnets:

permit 192.168.0.0 0.0.254.255

Note that neither of the wildcard masks above are contiguous; rather than matching a length from one side of the address to the other, each bit is matched (0) or ignored (1) independently.

A parting tip: you can determine the wildcard to match a subnet by subtracting each subnet mask octect from 255. Examples for /25 and /18 subnets are given below:

255 255 255 255

- 255 . 255 . 255 . 128 (/25)

-----------------------

0 . 0 . 0 . 127

255 255 255 255

- 255 . 255 . 192 . 0 (/18)

-----------------------

0 . 0 . 63 . 255

subnet (subnetwork)

A subnet (short for "subnetwork") is an identifiably separate part of an organization's network. Typically, a subnet may represent all the machines at one geographic location, in one building, or on the same local area network (LAN). Having an organization's network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate subnetworks, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. It would also require that Internet routing tables on gateways outside the organization would need to know about and have to manage routing that could and should be handled within an organization.

The Internet is a collection of networks whose users communicate with each other. Each communication carries the address of the source and destination networks and the particular machine within the network associated with the user or host computer at each end. This address is called the IP address (Internet Protocol address). This 32-bit IP address has two parts: one part identifies the network (with the network number) and the other part identifies the specific machine or host within the network (with the host number). An organization can use some of the bits in the machine or host part of the address to identify a specific subnet. Effectively, the IP address then contains three parts: the network number, the subnet number, and the machine number.

The standard procedure for creating and identifying subnets is provided in Internet Request for Comments 950.

The 32-bit IP address is often depicted as a dot address (also called dotted quad notation) - that is, four groups

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(or quads) of decimal numbers separated by periods. Here's an example:

130.5.5.25

Each of the decimal numbers represents a string of eight binary digits. Thus, the above IP address really is this string of 0s and 1s:

10000010.00000101.00000101.00011001

As you can see, we inserted periods between each eight-digit sequence just as we did for the decimal version of the IP address. Obviously, the decimal version of the IP address is easier to read and that's the form most commonly used.

Some portion of the IP address represents the network number or address and some portion represents the local machine address (also known as the host number or address). IP addresses can be one of several classes, each determining how many bits represent the network number and how many represent the host number. The most common class used by large organizations (Class B) allows 16 bits for the network number and 16 for the host number. Using the above example, here's how the IP address is divided:

<--Network address--><--Host address--> 130.5 . 5.25

If you wanted to add subnetting to this address, then some portion (in this example, eight bits) of the host address could be used for a subnet address. Thus:

<--Network address--><--Subnet address--><--Host address--> 130.5 . 5 . 25

To simplify this explanation, we've divided the subnet into a neat eight bits but an organization could choose some other scheme using only part of the third quad or even part of the fourth quad.

Once a packet has arrived at an organization's gateway or connection point with its unique network number, it can be routed within the organization's internal gateways using the subnet number. The router knows which bits to look at (and which not to look at) by looking at a subnet mask, which is a screen of numbers that tells you which numbers to look at underneath. In a binary mask, a "1" over a number says "Look at the number underneath"; a "0" says "Don't look." Using a mask saves the router having to handle the entire 32 bit address; it can simply look at the bits selected by the mask.

IP address (Internet Protocol Address)

This definition is based on Internet Protocol Version 4. See Internet Protocol Version 6 (IPv6) for a description of the newer 128-bit IP address. Note that the system of IP address classes described here, while forming the basis for IP address assignment, is generally bypassed today by use of Classless Inter-Domain Routing (CIDR) addressing.

In the most widely installed level of the Internet Protocol (IP) today, an IP address is a 32-bit number that identifies each sender or receiver of information that is sent in packets across the Internet. When you request an HTML page or send e-mail, the Internet Protocol part of TCP/IP includes your IP address in the message (actually, in each of the packets if more than one is required) and sends it to the IP address that is obtained by looking up the domain name in the Uniform Resource Locator you requested or in the e-mail address you're sending a note to. At the other end, the recipient can see the IP address of the Web page requestor or the e-mail sender and can respond by sending another message using the IP address it received.

An IP address has two parts: the identifier of a particular network on the Internet and an identifier of the particular device (which can be a server or a workstation) within that network. On the Internet itself - that is, between the router that move packets from one point to another along the route - only the network part of the address is looked at.

The Network Part of the IP Address

The Internet is really the interconnection of many individual networks (it's sometimes referred to as an internetwork). So the Internet Protocol (IP) is basically the set of rules for one network communicating with any other (or occasionally, for broadcast messages, all other networks). Each network

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must know its own address on the Internet and that of any other networks with which it communicates. To be part of the Internet, an organization needs an Internet network number, which it can request from the Network Information Center (NIC). This unique network number is included in any packet sent out of the network onto the Internet.

The Local or Host Part of the IP Address

In addition to the network address or number, information is needed about which specific machine or host in a network is sending or receiving a message. So the IP address needs both the unique network number and a host number (which is unique within the network). (The host number is sometimes called a local or machine address.)

Part of the local address can identify a subnetwork or subnet address, which makes it easier for a network that is divided into several physical subnetworks (for examples, several different local area networks or ) to handle many devices.

IP Address Classes and Their Formats

Since networks vary in size, there are four different address formats or classes to consider when applying to NIC for a network number:

Class A addresses are for large networks with many devices.
Class B addresses are for medium-sized networks.
Class C addresses are for small networks (fewer than 256 devices).
Class D addresses are multicast addresses.

The first few bits of each IP address indicate which of the address class formats it is using. The address structures look like this:

Class A

Class B

Class C

Class D

The IP address is usually expressed as four decimal numbers, each representing eight bits, separated by periods. This is sometimes known as the dot address and, more technically, as dotted quad notation. For Class A IP addresses, the numbers would represent "network.local.local.local"; for a Class C IP address, they would represent "network.network.network.local". The number version of the IP address can (and usually is) represented by a name or series of names called the domain name.

The Internet's explosive growth makes it likely that, without some new architecture, the number of possible network addresses using the scheme above would soon be used up (at least, for Class C network addresses). However, a new IP version, IPv6, expands the size of the IP address to 128 bits, which will accommodate a large growth in the number of network addresses. For hosts still using IPv4, the use of subnets in the host or local part of the IP address will help reduce new applications for network numbers. In addition, most sites on today's mostly IPv4 Internet have gotten around the Class C network address limitation by using the Classless Inter-Domain Routing (CIDR) scheme for address notation.

Relationship of the IP Address to the Physical Address

The machine or physical address used within an organization's local area networks may be different than the Internet's IP address. The most typical example is the 48-bit Ethernet address. TCP/IP includes a facility called the Address Resolution Protocol (ARP) that lets the administrator create a table that maps IP addresses to physical addresses. The table is known as the ARP cache.

Static versus Dynamic IP Addresses

The discussion above assumes that IP addresses are assigned on a static basis. In fact, many IP addresses are assigned dynamically from a pool. Many corporate networks and online services economize on the number of IP addresses they use by sharing a pool of IP addresses among a large number of users. If you're an America Online user, for example, your IP address will vary from one logon session to the next because AOL is assigning it to you from a pool that is much smaller than AOL's base of subscribers.

Internet

The Internet, sometimes called simply "the Net," is a worldwide system of computer networks - a network of networks in which users at any one computer can, if they have permission, get information from any other computer (and sometimes talk directly to users at other computers). It was conceived by the Advanced Research Projects Agency (ARPA) of the U.S. government in 1969 and was first known as the ARPANet. The original aim was to create a network that would allow users of a research computer at one university to be able to "talk to" research computers at other universities. A side benefit of ARPANet's design was that, because messages could be routed or rerouted in more than one direction, the network could continue to function even if parts of it were destroyed in the event of a military attack or other disaster.

Today, the Internet is a public, cooperative, and self-sustaining facility accessible to hundreds of millions of people worldwide. Physically, the Internet uses a portion of the total resources of the currently existing public telecommunication networks. Technically, what distinguishes the Internet is its use of a set of protocols called TCP/IP

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(for Transmission Control Protocol/Internet Protocol). Two recent adaptations of Internet technology, the intranet and the extranet, also make use of the TCP/IP protocol.

For many Internet users, electronic mail (e-mail) has practically replaced the Postal Service for short written transactions. Electronic mail is the most widely used application on the Net. You can also carry on live "conversations" with other computer users, using Internet Relay Chat (IRC). More recently, Internet telephony hardware and software allows real-time voice conversations.

The most widely used part of the Internet is the World Wide Web (often abbreviated "WWW" or called "the Web"). Its outstanding feature is hypertext, a method of instant cross-referencing. In most Web sites, certain words or phrases appear in text of a different color than the rest; often this text is also underlined. When you select one of these words or phrases, you will be transferred to the site or page that is relevant to this word or phrase. Sometimes there are buttons, images, or portions of images that are "clickable." If you move the pointer over a spot on a Web site and the pointer changes into a hand, this indicates that you can click and be transferred to another site.

Using the Web, you have access to millions of pages of information. Web browsing is done with a Web browser, the most popular of which are Microsoft Internet Explorer and Netscape Navigator. The appearance of a particular Web site may vary slightly depending on the browser you use. Also, later versions of a particular browser are able to render more "bells and whistles" such as animation, virtual reality, sound, and music files, than earlier versions.

Related glossary terms: HTTP (Hypertext Transfer Protocol), static analysis (static code analysis), intranet, Windows Workflow Foundation (WF or WinWF), XAML (Extensible Application Markup Language), scripting language, GUI (graphical user interface), Windows Communication Foundation (WCF), Windows Presentation Foundation (WPF), Windows File System (WinFS)

link\|improve this answer	answered Oct 13 '09 at 14:55 Pascal Thivent 1,273310
	Argh! Not binary! slits wrist – joshhunt Oct 13 '09 at 15:03

3
In practice no one uses classes anymore. It's usually only taught as a history lesson. CIDR is used instead now en.wikipedia.org/wiki/Classless_Inter-Domain_Routing – MarkM Oct 13 '09 at 15:08
	don't know that CIDR is any simpler to understand... :) – quack quixote Oct 13 '09 at 15:44
	It doesn't matter if it's simpler or not. Classes are no longer used and CIDR is. – MarkM Oct 13 '09 at 16:06

1
You can still operate on the Class boundaries and have your network function normally but it is antiquated. CIDRs of /8 /16 and /24 represent what Class A, B and C used to. – MarkM Oct 13 '09 at 18:32
	Getting started with subnets
To explore how subnets are used in the enterprise, here are some additional resources:
How to subnet: Subnetting calculations and shortcuts: Whether you need a subnetting cheat sheet or you're preparing for your CCNA exam, this tip and quiz on subnetting shortcuts shows you how to calculate a subnet mask, breaks down IP address classes, and explains binary and how to get bit values of one octet. If you're wondering how to figure out what subnet mask to use when you need "x" hosts and "x" networks, this tutorial shows you.
IP addressing and subnetting: What network administrators need to know: IP addressing and subnetting are an important part of networks. Learn about how IP addresses work, subnet masks, classful vs. classless IP addresses, default gateways and more.
Calculating subnets for IP addresses in IPv6 : Understand how subnetting in IPv6 differs from that of IPv4 and what you need to do in order to calculate the subnet of an IP address in IP version 6.
0	Network (7 bits)	Local address (24 bits)
10	Network (14 bits)	Local address (16 bits)
110	Network (21 bits)	Local address (8 bits)
1110	Multicast address (28 bits)