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IP Address & Subnetting
Transcript of IP Address & Subnetting
A class A address could be diagramed:
| | | |
which shows the eight network bits followed by the 24 host bits.These 32-bit IP addresses are almost always written as four dot-separated decimal numbers, one for each byte of the address. Thus, our class A address would have a range of address numbers from 18.104.22.168 through 22.214.171.124 ( 0. x.x.x and 127. x.x.x are reserved). The number of host addresses per network is 16,777,214, which is two less than two raised to the 24th power because both host numbers 0.0.0 and 255.255.255 are reserved.In practice, people don't really attach 16 million hosts to a network so administrators of a Class A site often divide the host address portion into a (sub)network and host portion. (Subnetting is now supported by most operating systems.) Each Class A network number can support up to 65,534 subnets (network numbers 0.0 and 255.255 are reserved) with each having 254 hos ts (host numbers 0 and 255 are reserved). This is done by using the 16 high -order bits of the host portion for the subnet number and the lower eight bits for the host as diagramed here:
Network Subnet Host
+------+ +--------------+ +------+
[0xxxxxxx][xxxxxxxxxxxxxxxx][xxxxxxxx] Subnet Addressing
The issue of subnetting has long been a mystery to many system administrators. It seems that there are just too many bits, bytes, and masks involved to make it worthwhile. Besides, who needs all that trouble when it's all you can do to just maintain the network as it is? You can certainly find sympathy, but if your network plans call for Internet access then IP (Internet Protocol) addressing and possibly subnet masking are topics that you should be familiar with.As networks grow to increasing numbers of segments, more network address numbers will be needed as each segment requires an individual number. The InterNIC (whose name signifies cooperation of Network Information Centers or NICs), however, is not eager to hand out unlimited network addresses because they are quickly running out of them. The Internet community is generally taking a firm stand on limiting address availability, due to the accelerated growth demand for Internet access. Network administrators will have to work with what they have to better accommodate network requirements and the shrinking supply of address numbers.One way of accomplishing this address conservation is to take the address that is assigned to your network and expand the capacity with subnets. Subnetting allows you to increase the number of networks available to you without applying for another IP address number. IP Addressing:
The IP addressing framework allows one to address about 16,000,000 unique hosts for a Class A address, around 65,000 hosts for a Class B address, but only 254 hosts for a Class C address. However, there are no more Class A addresses available, and the InterNIC has stopped assigning Class B addresses. Class C addresses are the most numerous, but their limitation is that each can identify only 254 unique hosts.The IP address is composed of 32 bits, which consist of two parts: the most significant bits (MSBs) identify a particular network and the remaining bits specify a host on that network. The most significant bits of the network portion actually determine the address class as shown in this table:
Class A 0
Class B 10
Class C 110 The Subnet Mask
A subnet mask (or number) is used to determine the number of bits used for the subnet and host portions of the address. The mask is a 32-bit value that uses one-bits for the network and subnet portions and zero-bits for the host portion.Let's look at an example. Here we have a Class B address of 126.96.36.199 and apply some different subnet masks. A logical AND operation is performed between the IP address and the subnet mask as shown:Here we use a mask that retains the default 16 network and host bits for a Class B address:
191 70 55 130
1011 1111 1000 0110 0011 0111 1000 0010
1111 1111 1111 1111 0000 0000 0000 0000
1011 1111 1000 0110 0000 0000 0000 0000
ResultHere we employ a mask that d ivides the host portion into a subnet and host that are each eight bits wide:
191 70 55 130
1011 1111 1000 0110 0011 0111 1000 0010
1111 1111 1111 1111 1111 1111 0000 0000
1011 1111 1000 0110 0011 0111 0000 0000
ResultThis division allows 254 (256-2 reserved) subnets, each with 254 hosts.This division on a byte boundary makes it easy to determine the subnet and host from the dotted-decimal IP address. However, the subnet-host boundary can be at any bit position in the host portion of the IP address. Here, we use a mask that allows more subnets (512-2 reserved), but with the trade-off of fewer hosts (128-2) per subnet:
191 70 55 130
1011 1111 1000 0110 0011 0111 1000 0010
IP address1111 1111 1111 1111 1111 1111 1000 0000
1011 1111 1000 0110 0011 0111 1000 0000
Result The Subnet Advantage
Subnetting hides the internal network organization to external routers and thus simplies routing. For instance, a subnetted Class B address would require fewer routes than the equivalent number of Class C addresses. Shorter routing tables mean faster network transfers.Subnetting allows address administration to be decentralized.
Besides technical advantages, this approach may also provide political benefits for the organization. For instance, an administrator could assi gn a subnet to a department, which would then be responsible for their own network management.Subnetting can help overcome distance limitations of physical networks by dividing up a physical network into individually addressed networks so they can be connected logically with routers. Fundamental There are three things I would like you to remember before we delve into subnetting.
1. If the host bits in a given IP address are all set to
'0', this is the network or subnet address.
2. If the host bits in a given IP address are all set to
'1', this is the broadcast address (all hosts in
the subnet/network are destination). Rule 2
The formula used to calculate the number of available subnets given the specific length of network mask. In order to determine the number of subnet bits to use them as the exponent in the above formula (Rule 1), you must first know what is the default network mask of the IP address according to its class. Then, you must count the bits that were added to this default network mask. Check out the below example. In the previous example , IP address belongs to class C since the first byte value is 192 . Class C uses first three bytes (24 bits) to denote the network portion of the address. Today we can say that its default network mask has the length of 24 bits (255.255.255.0). Since our network mask length is /28, we have extended the default network mask by 4 bits (bits in the green color). Thus, we get 4 subnet bits that must be used in our formula presented in Rule 1. Using the same example: 192.168.1.0/28, how many host addresses per subnet can we use?IP address table shows us that with /28 we have 4 bits left for host (total number of bits = 32). In order to calculate the available number of host addresses we must resort to formula presented in Rule 2.Make sure you understand how the three rules presented here work. Process Of subnetting In this process we will use the following as an example. There is a class B address. We require 6 subnets. Default subnet mask for Class B address: 255.255.0.0. Step One: Defining Subnet Mask
We now follow the following steps to define the subnet mask:
a) We have determined the number of physical segments in
our network (six). Convert this number to binary format: 110.
b) Count the number of bits required to represent the
number of physical segments in binary. Representing
six in binary requires three bits.
c) We need to borrow these three bits from the host
id section of the IP addresses.
d) Convert the required number of bits to decimal format
in high order (from left to right) in the third octet of the
default subnet mask.
For example, in this case, three bits are required; therefore,
configure the first three bits of the host ID (third octet of the IP address)
as the subnet ID. Our third octet of subnet mask is 1110000. The decimal
value for this is 224. Therefore the subnet mask for this
class B address is: 255.255.224.0 Classful Address Table Before we start using the above rules, let me show you a few examples of network, subnet and broadcast addresses based on what we have discussed in the last three posts including this one. If you do not remember the 'first octet rule', which determines the class and the default network mask of an IP address, use the following table as the reference. The number ranges of the first byte determine the classes as shown in picture. Subnetting process- Videos IP Address & Subnetting Activities Lab-exercises Activities
(Tutorial) You can use the following link to access the subnet tutorial :
(Survey) Here is a small survey to measure your understanding for subnetting. Use this link to attempt the survey:
http://www.surveymonkey.com/s/KRFZW98 Message Board You can use the following link to post your comments or issues or asking any question.
Expecting you to post something useful which is related to the this lesson. This is the link to the message board. Please go to button - Register To Post Messages - to register in Messageboard.
http://members.boardhost.com/ICTcourse/ Evaluation This section use to evaluate all resources and methods of teaching in this course as been demonstrated here. Conclusion This lesson provide the basic technique used to do subnetting for any network. It also increase your ability to produce more highly organized network through using these methods presented here. The learning outcome is should met with the evaluation criteria which has been set for this course. Therefore, you should be able as a student identified the type of network. Also You should be able to do subnetting and be able to design the proper network according to the requirement. In addition, You should recognized and detect the error or problem that could be occurred due to use of subnetting. Resources links:
http://uw714doc.sco.com/en/NET_tcpip/SetUpSubnetsRouters.html videos: Class B Addresses
The first two bits of a Class B address are 1 and 0, the next fourteen
bits identify the network and the last sixteen the host, as diagramed: Network Host
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Thus, Class B addresses include the network numbers in the range
from 188.8.131.52 through 184.108.40.206 for a total of 65,534 host addresses.As with the Class A address, we can divide the host portion of a Class B
address into subnet and host parts. For instance, let's spli t our Class B
network number on the byte boundary, that is, the eight MSBs of the
host portion identifies the subnet and the remaining bits the host,
Network Subnet Host
+--------------+ +------+ +------+
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This arrangement allows 254 subnets each with 254 hosts. Rule 3The formula used to calculate the number of available hosts per subnet or network given the specific network mask. .....continue Fundamental Step two: Defining Subnet IDs.
e) List the number of bits in high order used for the subnet ID. In this case 3 bits are to represent the subnet mask, the binary octet is 11100000. f)
Raise 2 to the power of the bit, and then subtract 2 to determine the possible bit combinations. Why subtract 2? Because we cannot have all 0s and all 1s subnets. g) Convert the bit with the lowest value to decimal format.
In this case we have 11100000. The lowest value is 32. h) Starting with zero, increment the value of each bit combination until the next increment is 256. i)
Defining subnet numbers In general, there are eight subnets (two invalid).
They will be numbered 0 through 7.
In other words, if we have to define subnet #n, we need to place the binary representation of n into the bits of the subnet-number field. For example, to define
subnet #6, simply place the binary representation of 6 (110) into the 3-bits of the subnet-number field.
Subnet 0: x.y.0.0 (All 0s not possible)
Subnet 1: x.y.32.0 Subnet 2: x.y.64.0
Subnet 3: x.y.96.0 Subnet 4: x.y.128.0
Subnet 5: x.y.160.0 Subnet 6: x.y.192.0
Subnet 7: x.y.224.0 (All is not possible) Step Three: Defining Host Ids
j) The result of each incremented value indicates the beginning of a range of host IDs for a subnet. If you increment the value one extra time, you can determine the end of the range (One less than the subnet mask)
k) The following table gives valid range of host IDs for this class B subnet using 3 bits for the subnet mask.
Bit Values Decimal Values Beginning Values Ending Values
-------------- ---------------------- ------------------------- ---------------------
00000000 0 Invalid
00100000 32 x.y.32.1 x.y.63.254
01000000 64 x.y.64.1 x.y.95.254
01100000 96 x.y.96.1 x.y.127.254
10000000 128 x.y.128.1 x.y.159.254
10100000 160 x.y.160.1 x.y.191.254
11000000 192 x.y.192.1 x.y.223.254
11100000 224 Invalid Invalid
l) To determine the number of hosts per subnet, raise 2 to the power of number of host ID bits, and then subtract 2. This is because the host number field of an IP address cannot contain all 0 bits or all 1 bits. In this case, 213-2=8190 hosts per subnet.
- The all-0s host number identifies the base network (or subnetwork) number), while
- all-1s host number represents the broadcast address for the network (or subnetwork). m) In general, to define the address assigned to host #n of a particular subnet, simply place the binary representation of n into the subnet’s host-number field. For example, to define the address, assigned to host 15 on subnet 3, simply place the binary representation of 15 (01111) into the allocated bits of subnet 3’s host number field.
Similarly, host 8190: x.y.011 11111.11111110 Step Four:
Defining broadcast address for each subnet
n) The broadcast address for a subnet #n is the all 1’s host address. For example, the broadcast address for subnet #3 is:
x.y.01111111.11111111 1 2 3 4 5 You should use the following pages to solve this exercises or use this link:
http://www.keepandshare.com/doc/5681731/subnetting-exercises-pdf-27k?da=y End of lecture. Prepared by SAID Additional Activities Games:
Scatter: IP Subnetting, Variable Length Subnet
Scatter: Subnet masks
Test: Exam Essentials - Subnetting, Variable Length Subnet Masks (VLSMs), and Troubleshooting TCP/IP
Test:IPv4 Network Classes & Reservations
Learn: IPv4 Network Classes & Reservations
Test: IP Subnetting, Variable Length Subnet Masks (VLSMs), & Troubleshooting IP