Subnetting

IntroductionSubnetting is the foundation underlying the expansion of both Local Networks & the Internet in todays world.Subnetting has become essential knowledge for the Administrator of any network.There are 2 fundamental reasons why subnetting has so much importance in todays networking environment:

Introduction (Cont.)The world is running out of available IP addresses. There just isnt an unlimited number of IP addresses available & subnetting helps extend the existing addresses until either the next version of IP is rolled out or some other technology charges on the scene.

Introduction (Cont.)Subnetting reduces the size of the routing tables stored in routers. Subnetting extends the existing IP address base & restructures the IP address. As a result, routers must have a way to extract from a IP address both the Network address & the Host address.

OctetsThe 32-bit IP address is broken up into 4 octets, which are arranged into a dotted-decimal notation scheme.An octet is a set of 8 bits & not a musical instrument.Example of an IP version 4:172.64.126.52

Thinking in BinaryThe binary system uses only 2 values 0 & 1 to represent numbers in positions representing increasing powers of 2.We all are accustomed to thinking & working in the decimal system, which is based on the number 10.

Thinking in Binary (Cont.)To most humans, the number 124 represents 100 + 20 + 4.To the computer, this number is 1111100, which is 64 (26) + 32 (25) + 16 (24) + 8 (23) + 4 (22) + 0 + 0

Each position in a binary number represents, right to left, a power of two beginning with 20 & increasing by one power as it moves left: 20, 21, 22, 24, etc.

Converting to DecimalYoull need to convert binary to decimal & vice versa to compute subnets & hosts.So, its time for a quick review lesson in binary-to-decimal conversion.There are 8 bits in an octet & each bit can only be a 1 or a 0.

Converting to Decimal (Cont.)What then do you suppose is the largest decimal number that can be expressed in an octet?

Eight 1s (1111 1111)

Converting to Decimal (Cont.)Now, for double the money, what is its equivalent decimal value?

Converting to Decimal (Cont.)Therefore, the largest decimal number that can be stored in an IP address octet is 255.The significance of this should become evident later in this presentation.

Classify and Define IPv4 Addresses3 types of addresses determined by HOST valueRange of addresses between Network & Broadcast addresse.g. 10.0.0.1 10.0.0.254

Determining the NW & Host portionsIndicated by the prefix or subnet mask (more later)E.g. network with prefix of /24 means24 bits used for NW portion8 bits used for Host portion (32 24)

Using all this now to work out the addresses for a given networkFor the address 172.16.20.0 with prefix /26What is the NW address?What is the Broadcast address?What is the range of Host addresses?

Using all this now to work out the addresses for a given network

For the address 172.16.4.32 with prefix /28What is the NW address?What is the Broadcast address?What is the range of Host addresses?172.16.4.32172.16.4.47172.16.4.33-46

Challenge with IPv4 addressI have 2 departments in my organizationEach has about 125 hostsI would like to keep them in separate NWsI have been assigned an IP network address space of 192.168.1.0 /24This is one NW, and I need 2I cannot change the IP address NW portionBut I can change the host portion

Solution = Subnetting (dividing network into separate networks)- Borrowing bits from the Host portion

Subnetting192.168.1.0 /2424 bit is NW portion8 bits is Host portionI can borrow from the Host portionHow many bits?2n # of subnets requiredn = # of bits to borrowSo for our example we need to borrow:2n 2Therefore n=1 (we need to borrow 1 bit)

SubnettingThe borrowed bits become part of my subnet mask/prefix (we borrowed 1 bit)Before subnetting /24 (255.255.255.0)After subnetting /25 (255.255.255.128)25 bit is NW portion & 7 bits is Host portionHow many hosts can I have per subnet?2n 2n = # of bits in the Host portionSo for our example:2n 2n=7Therefore we can have 126 hosts (128 2)(We needed 125)

SubnettingOur previous network was 192.168.1.0 /24How does our new networks look like?192.168.1.00000000 /25 or 192.168.1.0 /25192.168.1.10000000 /25 or 192.168.1.128 /25And there special addresses? Network/subnet 192.168.1.0 /25Broadcast 192.168.1.01111111 or 192.168.1.127Host range 192.168.1.00000001 01111110 or .1 - .126So how about the other subnet you calculate:Network/subnet 192.168.1.128 /25Broadcast 192.168.1.11111111 or 192.168.1.255Host range 192.168.1.10000001 11111110 or .129 - .254

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