business data communications

Business Data Communications

Addressing at the NL and TL

Review: Addressing at the DLL

DLL addresses are generally assigned to physical devices when they are manufactured.

Expressed in hexadecimal. An Ethernet address example:

00-0C-00-F5-03-5A DLL addresses identify hardware on a

network.

Addressing at the NL and TL

NL addresses are logical concepts. They uniquely identify sources and

destinations but not necessarily specific machines.

They are mapped to hardware addresses but not permanently. They can be changed.

The IP Address – IPv4 Example: 127.61.17.24 It represents a long binary number divided into four

parts. Each part is of eight bits so each part can assume the

values 0-255 since binary 00000000 = 0 and binary 11111111 = 255.

Called a “dotted quad”. Remember, computers can only deal with bits! How many bits does this address represent? How ever

many, it’s not enough…

Address classes

There is a large but finite number of addresses in this scheme. So some organization is necessary.

We can use the first bit to split all possible addresses into two groups. 00000000.00000000.00000000.00000000 to

01111111.11111111.11111111.11111111 10000000.00000000.00000000.00000000 to

11111111.11111111.11111111.11111111

Address classes - A

Consider the first group: 00000000.00000000.00000000.00000000 to

01111111.11111111.11111111.11111111 In decimal that is 0.0.0.0 to 127.255.255.255.

These addresses are called class “A”. About half of all possible addresses are of

this class. There are some other rules we’ll consider

later.

Address classes

Now consider that second group These are the ones that all start with 1. 10000000.00000000.00000000.00000000 to

11111111.11111111.11111111.11111111 Let us divide all these possible addresses into two

groups by using the second bit, giving… 10000000.00000000.00000000.00000000 to

10111111.11111111.11111111.11111111 11000000.00000000.00000000.00000000 to

11111111.11111111.11111111.11111111

Address classes - B

Consider the first of these groups: 10000000.00000000.00000000.00000000 to


These addresses are called class “B”.About one fourth of all possible

addresses are of this class (half of half).

Address classes

Now consider that second second group These are the ones that all start with 11. 11000000.00000000.00000000.00000000 to

11111111.11111111.11111111.11111111

Let us divide all these possible addresses into two groups by using the third bit, giving… 11000000.00000000.00000000.00000000 to

11011111.11111111.11111111.11111111 11100000.00000000.00000000.00000000 to

11111111.11111111.11111111.11111111

Address classes - C

Consider the first of these groups: 11000000.00000000.00000000.00000000 to


These addresses are called class “C”.About one eighth of all possible

addresses are of this class (half of half of half).

Address classes – other

Finally, consider the second of these groups: 11100000.00000000.00000000.00000000 to


These are the other class addresses that are not in general use.

Address Classes Summary The first bit distinguishes Class A from the rest. Class A

addresses start with 0. 00000000.00000000.00000000.00000000 to

01111111.11111111.11111111.11111111 Class B addresses start with 1. The second bit, being 0,

distinguishes Class B, from the rest. 10000000.00000000.00000000.00000000 to

10111111.11111111.11111111.11111111 Class C addresses start with 11. The third bit, being 0,

distinguishes Class C from the rest. 11000000.00000000.00000000.00000000 to

11011111.11111111.11111111.1111111

Assigned Class Addresses

So we have this huge range of addresses called “Class A” to assign to people… 00000000.00000000.00000000.00000000 to

01111111.11111111.11111111.11111111

Q: How do we organize this? A: The number assigning authority decrees that it

will assign addresses by fixing the first byte and letting you play with the rest how you will.


Class A addresses: 1st byte is assigned to you. Class B addresses: First 2 bytes are assigned to

you. Class C addresses: First 3 bytes are assigned to

you. Here’s a sample Class A address: 115.x.x.x.

You know it’s class A because of the value of the first byte. The values of the other three bytes are up to you to assign

to the devices on your network.


Class B addresses: First 2 bytes are assigned to you. Here’s a sample Class B address: 152.15.x.x. You know it’s class B because of the value of the first byte. The values of the other two bytes are up to you to assign to

the devices on your network


Class C addresses: First 2 bytes are assigned to you. Here’s a sample Class B address: 200.18.12.x. You know it’s class C because of the value of the first byte. The values of the last byte are up to you to assign to the

devices on your network.

Subnetting

Just as the IANA divides up all the available addresses in a class by assigning fixed values to bytes, we can do the same within our own allocation of addresses.

How we do this would depend on how many subnets we wanted to create in our network. Maybe subnet by department or building or floor

or whatever…

Subnetting

For example, if we’re given the class B address 152.15.x.x, we have two bytes to play with mostly however we want.

It’s up to us to divide those bytes among subnets and the actual equipment on the network, called hosts.

We denote the use of bytes using the letters n(etwork), s(ubnet), and h(ost). 152.15.x.x goes to nnnnnnnn.nnnnnnnn.xxxxxxxx.xxxxxxxx

(for now)

Subnetting 152.15.x.x

We control all addresses from 152.15.0.0 to 152.15.255.255! Woohoo!

Assume the extreme case – we don’t want to subdivide our addresses. We denote this: nnnnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh.

This means that, on our one big network, we can have host addresses from x.x.0.0 to x.x.255.255, only you can’t have a host address of all 0s or all 1s so it’s really x.x.0.1 to x.x.255.254. (Hosts of all 0s and all 1s have special uses.)


Assume that we wanted to divide our network into two subnets. We could do this by using the first bit of the bytes that we control. We denote this: nnnnnnnn.nnnnnnnn.shhhhhhh.hhhhhhhh.

Setting this bit to 1 or 0 enables us to divide our addresses into two groups. This is the same system used to create classes.


Subnet one is: 152.15.0.1 to 152.15.127.254.

Subnet two is: 152.15.128.1 to 152.15.255.254

Remember that you can’t have hosts of all 0s or all 1s so 0 and 255 are right out.


Assume that we wanted to divide our network into four subnets. We could do this by using the first two bits of the bytes that we control. We denote this: nnnnnnnn.nnnnnnnn.sshhhhhh.hhhhhhhh.

Setting these bits to 00, 01, 10, or 11 enables us to divide our addresses into four groups.

Subnetting 152.15.x.x Subnet one is:

152.15.0.1 to 152.15.63.254. x.x.00000000.00000001 to x.x.00111111.11111110

Subnet two is: 152.15.64.1 to 152.15.127.254 x.x.01000000.00000001 to x.x.01111111.11111110

Subnet three is: 152.15.128.1 to 152.15.191.254 x.x.10000000.00000001 to x.x.10111111.11111110

Subnet four is: 152.15.192.1 to 152.15.255.254 x.x.11000000.00000001 to x.x.11111111.11111110


Consider our class B address 152.15.x.x. We decide to use five of the bits to designate

subnets: Nnnnnnnn.nnnnnnnn.ssssshhh.hhhhhhhh This gives us 25 or 32 subnets. Each subnet has 211 – 2 or 2046 host addresses. 32 x 2046 = 65,472 total addresses.


Consider our class B address 152.15.x.x. We decide to use seven of the bits to

designate subnets: Nnnnnnnn.nnnnnnnn.sssssssh.hhhhhhhh This gives us 27 or 128 subnets. Each subnet has 29 – 2 or 510 host addresses. 128 x 510 = 65,280 total addresses.

Subnet masking – basic concept

A mask is a bit pattern that we can use to indicate which parts are shared, are the same, among a group of IP addresses. This would indicate that those addresses are on the same part of the network.

A mask is an IP-address-like thing created by comparing the bits of two IP addresses, starting from the left. As long as the bits in the same position (1st, 2nd, etc) are alike, the corresponding bit in the mask is set to one. But the first time they differ, the corresponding bit in the mask is set to 0 and so are all the remaining bits in the mask.

Subnet masking – basic concept

For example, given two addresses, 152.15.12.9 and 152.15.46.48, we can create a mask that would accommodate them: 1011000.00001111.00001100.00001001 1011000.00001111.00101110.00110000

The mask would be: 1111111.11111111.11000000.00000000

or 255.255.192.0

Subnet masking – class mask

Given that mask, 255.255.192.0, why is it not surprising that the first two bytes are both 255?

Because these two addresses belong to the same class B network address! The mask for two addresses that are part of the same class B

network address will always begin with 255.255! And the mask for two addresses that are part of the same class

A network address will always begin with 255. And the mask for two addresses that are part of the same class

C network address will always begin with 255.255.255. This is called the class mask. Easy to see from the first byte.

Subnet masking – network mask

To addresses that are on the same subnet, will share another set of bits in common – those that you decided to use to designate subnets.

Consider that you decided to use seven of the bits to designate subnets: nnnnnnnn.nnnnnnnn.sssssssh.hhhhhhhh

The mask that identifies your subnets must be: 11111111.11111111.11111110.00000000 or 255.255.254.0

Subnet masking – use

A network manager wants to allow access to a file server only to requests that come from within the company’s network. The company has been given a class C address of 220.121.10.X. The mask is 255.255.255.0 because it is a class C address. The firewall can simply compare all source addresses of

requests to its own address, say 220.121.10.1 using the mask. So, given a request from 220.121.12.2: 11011100.01111001.00001100.00000010 = 220.121.12.2 11011100.01111001.00001010.00000001 = 220.121.10.1 11111111.11111111.11111111.00000000 = 255.255.255.0 yyyyyyyy.yyyyyyyy.yyyyynxx.xxxxxxxx = fails!

Subnet masking – use The same guy wants to allow access to a file server only to requests

that come from within a certain department that owns the firewall. The company has been given a class C address of 220.121.10.X and uses the first two bits to designate subnets. The mask is 255.255.255.192. The first three bytes are the class mask. 192

comes from the use of the first two bits of the fourth byte to designate subnets.

The firewall can simply compare all source addresses of requests to its own address, say 220.121.10.130 using the mask. So, given a request from 220.121.10.12: 11011100.01111001.00001010.10000010 = 220.121.10.130 11011100.01111001.00001010.00001100 = 220.121.10.12 11111111.11111111.11111111.11000000 = 255.255.255.192 yyyyyyyy.yyyyyyyy.yyyyyyyy.nxxxxxxx = fails!

IPv6

The problem with IPv4 is that there are not enough addresses.

IPv6 addresses this by providing long addresses, hence more potential addresses.

Downward compatible with IPv4.

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