Hands-On Microsoft Windows Server 2003 Networking Chapter Four Subnetting

2 Objectives Understand why subnetting is necessary Describe how computers work with binary TCP/IP addresses Subnet any network Determine the number of useable hosts on a subnet Supernet several smaller networks

3 Reasons to Subnet Subnetting –The process of separating a network into several smaller networks to improve performance Subnetting is used because it can –Reduce collisions –Limit broadcasts –Control traffic

4 Reducing Collisions Collision –When two computers on an Ethernet network using CSMA/CD as an access method attempt to transmit at the same time Subnetting –Reduces number of hosts on each network –Reduces the amount of traffic on the network –With less traffic, collision is reduced Collisions that occur on one network do not affect another network

5 Limiting Broadcasts Subnetting a network –Creates multiple networks with fewer hosts on each network –Fewer hosts results in fewer broadcast messages Routers –Packets addressed to IP address are not forwarded

6 Controlling Traffic Routers –Allows a greater degree of control over network traffic –Most have ability to implement rules about which packets they forward

7 Binary and TCP/IP IP addresses –Internally, computer looks at an IP address as a single group of 32 binary digits Decimal numbering –A base-ten numbering system –Uses the values from 0 to 9 for each digit

8 Counting in Decimal

9 Decimal Column Values

10 Binary Numbering Subnetting –Based on binary Binary –Base two numbering system –Two potential values for each digit: 0 and 1

11 Binary Counting

12 Binary Column Values

13 Conversion Between Binary and Decimal To convert a binary octet to a decimal value –Multiply the digit in each column by the value of each column –Then determine the sum of those products

14 Binary to Decimal Conversion

15 Windows Calculator in Scientific Mode

16 Binary Subnet Mask Computer calculates the host ID and network ID of an IP address while working in binary Part of the network ID –Where there is a 1 in the subnet mask Part of the host ID –Where there is a 0 in the subnet mask ANDing –Binary process used by computers to find the network ID –An IP address ANDed with a subnet mask gives the network ID 1s in a subnet mask must be contiguous

17 Calculating Host ID and Network ID

18 Invalid Subnet Masks

19 Subnetting a Network To subnet a network –Take some bits from the host ID and give them to the network ID Class B addresses –Normally very large –Generally need to be subnetted to handle routing between different physical locations Using an entire octet for subnetting gives 256 possible subnets Traditionally, subnets with all 1s and all 0s are discarded, leaving 254 usable subnets

20 Simple Subnetting

21 Complex Subnetting

22 Subnet Hosts 2 n -2, where n is the number of host bits –Formula used to calculate the number of subnets that can be created from a certain number of bits –Also used to calculate the number of usable hosts on a subnet

23 Usable Hosts

24 Usable Hosts Formula

25 Supernetting The opposite of subnetting Used to create one large network from several smaller ones Takes bits from network ID and gives them to the host ID All networks being combined for supernetting must be contiguous IP addresses from first network to last must be one single range with no breaks

26 Supernetting Two Class C Networks

27 Supernetting Four Class C Networks

28 Reasons for Supernetting Supernetting –Used when a range of IP addresses larger than a Class C network is required, but a full Class B network is not required –May also be used to reduce routing complexity

29 Summary Subnetting –Used to divide a single large network into multiple smaller networks –Reduces packet collisions –Limits broadcasts –Controls network traffic Binary –Base-two numbering system –Only 0 and 1 are valid values Computers –Work with IP addresses as 32-digit binary numbers

30 Summary (Continued) Subnet Mask –1 corresponds with bit that is part of network ID –0 corresponds with bit that is part of host ID Subnetting –Takes bits from host ID and uses them as part of network ID Number of useable hosts on a network –Calculated with the formula 2 n -2 –n is the number of host bits Supernetting –Combines multiple smaller networks into a single larger network –Networks being combined must be contiguous