CSN07102 Practical Networks 1

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1 CSN07102 Practical Networks 1 Subnetting Step-by-step 1. Make up quick reference/crib tables a. Binary weights b. Subnet Mask Values c. IP Addressing Crib Table F:\csn07102 PN1\F07 Subnet SbS.doc 04/04/ Figure out the network requirements a. Total number of networks b. The maximum number of interfaces attached to each. 3. Allocate address ranges to networks a. Largest network (most host interfaces) allocated first. b. Keep a note of the next available (unallocated) address. 4. Find the usable address range for each subnet 5. Allocate addresses to interfaces a. End devices (PCs/Servers) get the lowest numbers from the network range working upwards. b. Infrastructure devices (Routers) get highest numbers from the network range downwards. c. Assign the default gateway value on end devices. d. Assign the subnet mask value on all device interfaces. 1. Make up your quick reference tables Planning a subnet addressing scheme is not particularly difficult, there is no complex maths only doubling, halving, incrementing address values. However a typical subnet plan involves dozens of numerical values many of which are almost (but not quite) identical. It is therefore very easy to make a simple mistake in calculating or transcribing values even when the calculation is trivial. Since you will be carrying out many similar tasks for any subnet planning exercise it makes sense to do as much of the mundane calculation beforehand, creating your own quick reference tables. Transcribing errors in the tables are much more likely to be spotted as the numbers for the different parameters will follow an obvious pattern. An errors encountered whilst doing a one-off calculation will not be so obvious.

2 a. Binary weights This table gives the weight of each binary bit in an octet (or byte) in decimal, it is simply created by doubling each weight as you move from right (Bit 0) to left (Bit 7). Make up this table in a handy place as it is very useful for quick reference. BitNo Weight b. Subnet Mask Values Subnet mask values are 32 bit numbers which consist of a string of ones followed by zeros. The subnet mask for a network based on the familiar /24 prefix is 24 ones (3 octets) followed by 8 zeros (1 octet). The /24 subnet mask is the /25 subnet mask is the /26 subnet mask is etc. However we always write subnet mask values in dotted decimal form, so the above becomes:- /24 subnet mask is /25 subnet mask is /26 subnet mask is Therefore there are only eight dotted decimal values (from 255 to 0) that can ever appear in a subnet mask octet. These can all be reliably calculated from the binary weight table in a few seconds. Sum the binary weights from the left to get the values for all possible valid subnet mask octet values. i.e =192, =224, =240 etc. BitNo Subnet Mask Dec Bin

3 c. IP Addressing Crib Table The size of a subnet in terms of how many IP addresses it encompasses is defined by the network prefix. For example a subnet with a /24 prefix uses 256 IP addresses. The subnet mask value gives exactly the same information but expressed in dotted decimal form. We configure interfaces with IP addresses and subnet mask values but we usually define a network size using the corresponding prefix value. There is a one-to-one relationship between a network prefix and subnet mask value. The final quick reference table created will give all these key design parameters for the most common network prefixes. Start with the familiar standard network size (/24), where subnets can be allocated very simply by incrementing the 3 rd octet. Example: /24 (1 st subnet) /24 (2 nd subnet) /24 (3 rd subnet) etc. Network Prefix Subnet Mask (SM) Subnet Increments (256-SM) No of IPs Subnet Range (SR) Max Hosts (SR-2) / x256= Remember if the address range is 256 addresses, then the usable range which can be assigned to network devices is 2 less (254). This is because we cannot use the first address (all zeros) as it represents the network address and we cannot use the last address (all ones) as this is the broadcast address. Now simply calculate the parameters for networks twice this size (/23) and half this size(/25).

4 Network Prefix Subnet Mask (SM) Subnet Increments (256-SM) No of IPs Subnet Range (SR) Max Hosts (SR-2) / x256= / x256= / A /23 network covers twice as many IP addresses as a /24, whenever we allocate one the next would be 2 greater in the third octet (not 1 greater). So a subnet address sequence might be:- Example: /23 (1 st subnet) /23 (2 nd subnet) /23 (3 rd subnet) etc. A /25 network covers half the IP addresses of a /24 whenever we allocate one the next would be 1/2 greater or 128 in the fourth octet. So a subnet address sequence might be:- Example: /25 (1 st subnet) /25 (2 nd subnet) /25 (3 rd subnet) /25 (4 th subnet) /25 (5 th subnet) etc Repeat this process to complete the table for all network prefix sizes that you might expect to see in a configuration exercise. (e.g. /21 to /30) As a check you will find that if you add the subnet increment value to the corresponding octet in the subnet mask you will always get 256. Example: Subnet Increment Subnet mask = 256

5 2. Figure out the network requirements When designing an IP addressing plan from a network topology the most important thing is to identify how many separate (sub) networks are in the design. For the purposes of IP address planning networks are bounded by routers. Wherever you see a router interface this represents the edge of a network. Any non-layer 3 devices (e.g. switches, bridges, hubs or repeaters) can be ignored. The next stage is to consider how many unique IP addresses will be required by each of these separate networks, remembering to include all hosts and router interfaces. In a real life situation you may wish to consider future expansion, i.e. more PCs or servers being added at a later date. a. Total number of networks Count the number of separate networks in your given design, remember that networks are bounded by routers. Do not forget router-router serial links if they are part of your plan. b. The maximum number of interfaces attached to each. For each separate network, note the maximum number of device interfaces which will be attached, remember to include the router interfaces. 3. Allocate address ranges to networks Sort the list of networks by size, the one with the largest number of hosts first. a. Largest network (most host interfaces) allocated first. Take the largest network and, using the IP Addressing Crib Table, determine the network prefix from the required number of hosts. You will almost always have to allocate more addresses than you use. Network prefix and subnet mask values can be read straight from the crib table. The address range for this network will start at the beginning of the available address space provided for you. E.g. If you have been allocated /16 then your first subnet will be /--

6 b. Keep a note of the next available (unallocated) address. This is calculated by taking the network address and adding the subnet increment value from the crib table. For example, if your calculated subnet is /22, the next available (free) address is (128+4=132) (Note: /22 networks are allocated incrementally in steps of ) i. Take the next largest network and its network prefix, however this network s address range begins where the previous one finishes. So this network address will be the next available address as calculated previously. ii. Repeat the previous step with successively smaller networks until all subnets have been allocated the correct network address according to the prefixes selected. 4. Find the usable address range for each subnet Subnet 1 Network Address.128/27 First usable host.129 The usable address range is the range of IP addresses which can be used by host and router interfaces on this network. This starts immediately after the network address and ends immediately before the broadcast address. The easiest way to find this range is Last usable host Broadcast Address Subnet 2 Network Address just to add 1 to the network address to get the first usable host address and subtract 2 from the next subnet s address to get the last usable host address Allocate addresses to interfaces a. End devices ( lowest usable numbers upwards). The lowest number in the address range for the network that the interface is attached to is the Network Address and cannot be used for an interface, the next address (+1) is the first which can be allocated to a device. It is common practice to assign end devices (PCs, servers) IP addresses starting from the lowest usable numbers. b. Infrastructure devices (highest usable numbers downwards). The highest number in the address range is the Broadcast Address this cannot be allocated to an interface, the next one down (-1) is the first which can be allocated. It is common practice to assign a router interface the highest usable IP address for the network that it attaches

7 to, this will be the default gateway for the end devices on this network. c. Assign the default gateway value on end devices. Devices on the network use the attached router interface as their default gateway so this value will be the same for all end devices on a network particular subnet. d. Assign the subnet mask value on all device interfaces All device interfaces (end device and router) connected to the same (sub) network will have the same subnet mask value which is dictated by the network prefix used. Default Gateway for all Subnet C devices All device interfaces on the same subnet share the same subnet mask Default Gateway address Once all interfaces have IP addresses, subnet masks and default gateway values then the allocation is complete.

8 Handy IP Addressing Crib Table Network Prefix Subnet Mask (SM) Subnet Increments No of IPs Max Usable Hosts IPs (256 - SM) (SR-2) / x256= / x256= / x256= / x256= / / / / / /