Subnetting Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: 27 June 2002

Topics The Basics of Subnetting Subnet Mask Computing subnets and hosts Subnet Routing Creating a Subnet Example of Subnetting

Addressing without Subnets 172.16.1.2 172.16.1.3 172.16.2.1 172.16.254.254 172.16.0.0 A class B “Flat Network”, more than 65000 hosts How to manage? Performance?

Addressing with Subnets 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 172.16.1.0 172.16.2.0 172.16.4.2 172.16.4.3 172.16.3.2 172.16.3.3 172.16.3.0 172.16.4.0 Network administrators sometimes need to divide networks, especially large ones, into smaller networks. These smaller divisions are called subnetworks and provide addressing flexibility. Most of the time subnetworks are simply referred to as subnets. Similar to the host number portion of Class A, Class B, and Class C addresses, subnet addresses are assigned locally, usually by the network administrator. Also, like other IP addresses , each subnet address is unique. A class B “subdivided network”, smaller groups with routers

Subnetwork benefits Subnetwork Smaller networks are easier to manage and troubleshoot Increase the network manager's control over the address space Overall traffic is reduced, performance may improve Subnetwork A primary reason for using subnets is to reduce the size of a broadcast domain. Broadcasts are sent to all hosts on a network or subnetwork. When broadcast traffic begins to consume too much of the available bandwidth, network administrators may choose to reduce the size of the broadcast domain. Subdivide on IP network number is an important initial task of network managers

Subnet Address Before Subnetting Network ID Host ID After Subnetting Subnet ID Host ID To create subnets, you must extend the routing portion of the address. The Internet knows your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24 routing bits (the network number). The subnet field will become additional routing bits, so that the routers within your organization can recognize different locations, or subnets, within the whole network Subnet addresses include the Class A, Class B, or Class C network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion for the entire network. The ability to decide how to divide the original host portion into the new subnet and host fields provides addressing flexibility for the network administrator. To create a subnet address, a network administrator borrows bits from the original host portion and designates them as the subnet field. A subnet address is created by borrowing bit from the Host ID and designated it as a Subnet ID field

Define physical subnetwork Define individual hosts How to assign subnet Each class can have different size of subnet field Define physical subnetwork Define individual hosts Network Subnet Host choose appropriate size Figures and illustrate the hierarchical nature of subnet addresses.  To create a subnet address, a network administrator borrows bits from the host field and designates them as the subnet field. The minimum number of bits that can be borrowed is 2. If you were to borrow only 1 bit, to create a subnet, then you would only have a network number - the .0 network - and the broadcast number - the .1 network. The maximum number of bits that can be borrowed can be any number that leaves at least 2 bits remaining, for the host number. In this example of a Class C IP Address, bits from the host field for the subnet field have been borrowed.   Class Size of Default Host Field Max Number of Subnet Bits A 24 22 B 16 14 C 8 6 Previous standards did not allow for the use of subnets obtained by borrowing 1 bit  (with only 1 subnet bit, the subnet field can only have two values: subnet 0 is part of the network address, and subnet 1 would be part of the network broadcast address) – although many devices can now support subnets obtained by borrowing 1 bit, it is still common practice to avoid doing this to insure compatibility with legacy devices; for our purposes here, you will always borrow at least 2 bits. 2 Similarly, a 1 bit host field would allow only for host 0, which is part of the network address, and host 1, which is part of the broadcast address, leaving 0 valid host addresses Class A : 2 to 22 bits Class C : 2 to 6 bits Class B : 2 to 14 bits

Subnet Example Class B address such as 172.16.0.0 might use its third byte to identify subnet Subnet Network Address Address Range #1 172. 16. 1. 172.16.1.1-172.16.1.254 #2 172. 16. 2. 172.16.2.1-172.16.2.254 #3 172. 16. 3. 172.16.3.1-172.16.3.254 #254 172. 16. 254. 172.16.254.1-172.16.254.254

Subnet mask subnet mask is a 32 bit number, use to identify a subnet Example : A class B network with 24 bits mask 1111 1111 0000 0000 Network ID Subnet ID Host ID Set the bit covering the network and subnet ID to 1 1 255. 0. zero bit are used to mask out the host number resulting the network address 2 The subnet mask (formal term: extended network prefix), is not an address, but determines which part of an IP address is the network field and which part is the host field. A subnet mask is 32 bits long and has 4 octets, just like an IP address.  To determine the subnet mask for a particular subnetwork IP address follow these steps. (1) Express the subnetwork IP address in binary form. (2) Replace the network and subnet portion of the address with all 1s. (3) Replace the host portion of the address with all 0s. (4) As the last step convert the binary expression back to dotted-decimal notation. Note: The extended network prefix includes the class A, B, or C network number, plus the subnet field (or subnet number) that is being used to extend the routing information (which is otherwise just the network number). subnet mask= 255.255.255.0

Masking 172.16.4.2 & 255.255.255.0 1 1 1 172.16.4.0 The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet: the lowest numbered address is the address of the subnet. In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address. A “bitwise-and” between IP address and subnet mask yields a network address. Note that zeros bit are used to mask out the host number resulting the network address

Subnet mask in Prefix format The number of routing bits (network and subnet bits) in each subnet mask can also be indicated by the "/n " format.  255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 /8 255.255.192.0 1111 1111 1111 1111 1100 0000 0000 0000 /18 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 /24 255.255.255.240 1111 1111 1111 1111 1111 1111 1111 0000 /28 172.16.0.0 255.255.255.0 = 172.16.0.0/24

Subnet routing Traffic is routed to a host by looking “bit-wise and” results if dest_ip_addr & subnet_mask = = my_ip_addr & subnet_mask send pkt on local network %dest ip addr is on the same subnet else send pkt to router %dest ip addr is on diff subnet The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet: the lowest numbered address is the address of the subnet. In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address.

Routing Hosts and routers perform logical AND to send packets 1 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 To 172.16.4.2 1 172.16.1.0/24 172.16.2.0/24 2 172.16.4.2 172.16.4.3 172.16.3.2 172.16.3.3 3 172.16.3.0/24 172.16.4.0/24 The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet: the lowest numbered address is the address of the subnet. In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address. In the Figure, the router has received a packet for host 131.108.2.2 - it uses the AND operation to learn that this packet should be routed to subnet 131.108.2.0. The process of ANDing is explained in Lab 10.6.6. 172.16.1.3 has a packet for 172.16.4.2 and determine that it is on other subnetwork The packet is sent to the router The router performs a subnet masking and sends the packet to the destination network

Subnet interpretation IP Address subnet mask Interpretation 15.20.15.2 255.255.0.0 host 15.2 on subnet 15.20.0.0 130.122.34.3 255.255.255.192 host 3 on subnet 130.122.34.0 130.122.34.132 255.255.255.192 host 4th on subnet 130.122.34.128 158.108.2.71 255.255.255.0 host 71 on subnet 158.108.2.0 200.190.155.66 255.255.255.192 host 2nd on subnet 200.190.155.64

Default Subnet mask A default subnet mask : a subnet mask with no subnetting Class A 255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 Class B 255.255.0.0 1111 1111 1111 1111 0000 0000 0000 0000 By default, if you borrow no bits, the subnet mask for a Class B network would be 255.255.0.0, which is the dotted decimal equivalent of 1s in the 16 bits corresponding to the Class B network number. Class C 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000

Range of bit A default subnet mask : a subnet mask with no subnetting IP 172 16 Default subnet 255 255 New subnet To create subnets, you must extend the routing portion of the address. The Internet knows your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24 routing bits (the network number). The subnet field will become additional routing bits, so that the routers within your organization can recognize different locations, or subnets, within the whole network. By default, if you borrow no bits, the subnet mask for a Class B network would be 255.255.0.0, which is the dotted decimal equivalent of 1s in the 16 bits corresponding to the Class B network number. If 8 bits were to be borrowed for the subnet field, the subnet mask would include 8 additional 1 bits, and would become 255.255.255.0. For example, if the subnet mask 255.255.255.0 were associated with the Class B address 130.5.2.144 (8 bits borrowed for subnetting), the router would know to route this packet to subnet 130.5.2.0 rather than to just network 130.5.0.0 255 255 255 Define a subnet mask by extending the network portion to the right, 8 bits in this example

Computing subnet mask Decimal equivalents of bit patterns Binary mask 1 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 1 0 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 1 0 0 0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 128 192 224 240 248 252 254 255 Binary mask Octet value 128 64 32 16 8 4 2 1 Whenever you borrow bits from the host field, it is important to note the number of  additional subnets that are being created each time you borrow one more bit. You have already learned that you cannot borrow only 1 bit; the fewest you may borrow is 2 bits. Borrowing 2 bits creates four possible subnets (22) (but you must always remember that there are two reserved/unusable subnets). Each time you borrow another bit from the host field, the number of subnets created increases by a power of 2. The eight possible subnets that are created by borrowing 3 bits is equal to 23 (2 x 2 x 2). The sixteen possible subnets created by borrowing 4 bits is equal to 24 (2 x 2 x 2 x 2). From these examples, it is easy to see that each time you borrow another bit from the host field, the number of possible subnets doubles.

Compute Net and host How many subnet and host are there with 172.16.0.0/24 255. 0. 1111 1111 0000 0000 Network ID Subnet ID Host ID Each time you borrow 1 bit from a host field, there is 1 less bit remaining in the field that can be used for host numbers. Specifically, each time you borrow another bit from the host field, the number of host addresses that you can assign decreases by a power of 2 (gets cut in half). To help you understand how this works, use a Class C network address as an example. If there is no subnet mask, all 8 bits in the last octet are used for the host field. Therefore, there are 256 (28) possible addresses available to assign to hosts (254 usable addresses, after you subtract the 2 you know you can't use). Now, imagine that this Class C network is divided into subnets. If you borrow 2 bits from the default 8 bit host field, the host field decreases in size to 6 bits. If you write out all of the possible combinations of 0s and 1s that could occur in the remaining 6 bits, you would discover that the total number of possible hosts that could be assigned in each subnet would be reduced to 64 (26). The number of usable host numbers would be reduced to 62. In the same Class C network, if you borrow 3 bits, the size of the host field decreases to 5 bits and the total number of hosts that you could assign to each subnet would be reduced to 32 (25). The number of usable host numbers would be reduced to 30.  The number of possible host addresses that can be assigned to a subnet is related to the number of subnets that have been created. In a Class C network, for example, if a subnet mask of 255.255.255.224 has been applied, then 3 bits (224 = 11100000) would have been borrowed from the host field. The useable subnets created are 6 (8 minus 2), each having 30 (32 minus 2) useable host addresses. 8 bit subnet ID = 28=256 => 254 subnets 8 bit host ID = 28=256 => 254 hosts per subnet

Network and Host relationship Sample class C Number of subnet bits Number of subnets created Number of hosts per subnet Total number of hosts Percents used 2 62 124 49% 3 6 30 180 71% 4 14 196 77% 5 One of the decisions that you must make whenever you create subnets is to determine the optimal number of subnets and hosts (Note: The number of subnets required in turn determines the number of hosts available. For example, if you borrow 3 bits with a Class C network, only 5 bits remain for hosts). Lab Activity  This lab focuses on a Class C network with three subnets and using a Custom Subnet Mask. It will help you develop a better understanding of IP subnet masks. You have already learned that you cannot use the first and last subnet. You also cannot use the first and last address within each subnet - one is the broadcast address of that subnet, and the other is part of the network address. When you create subnets, you lose quite a few potential addresses. For this reason, network administrators must pay close attention to the percentage of addresses that they lose by creating subnets. Example: If you borrow 2 bits with a Class C network, you create 4 subnets, each with 64 hosts. Only 2 of the subnets are usable and only 62 hosts are usable per subnet, leaving 124 usable hosts out of 254 that were possible before you chose to use subnets. This means you are losing 51% of your addresses. Imagine, this time, that you borrow 3 bits. You now have 8 subnets, of which only 6 are usable, with 30 usable hosts per subnet. This gives you a total of 180 usable hosts, down from 254, but now you are losing only 29% of your addresses. Whenever you create subnets, you need to take into consideration future network growth and the percentage of addresses that you would lose by creating subnets.

Subnetting Special Addresses Reserved addresses that are not allowed to be assigned to any node NetID HostID Purpose any All 0s Subnetwork Address Example: 172.16.2.0/24 Subnetwork 172.16.2.0 any All 1s Subnet-directed Broadcast Example: 172.16.2.255/24 Directed broadcast of the subnetwork 172.16.2.0

Subnet Net Block Diagram No subnetting 2 bits Block diagram subnetting class C 3 bits 4 bits Network Address Broadcast Address

Contiguous and Noncontiguous mask no intermedite 0 gaps in the subnet mask Contiguous subnet mask 1111 1111 1111 1111 1111 1111 0000 0000 intermedite 0 gaps in the subnet mask Noncontiguous subnet mask 1111 1111 1111 1111 0001 1111 0000 0000 Noncontiguous leads to complex subnetting and routing It is strongly recommend to use contiguous subnet mask

Subnet Class A Example subnet mask Interpretation 255.0.0.0 1 network with 1677214 hosts (default subnet) 255.255.0.0 254 subnets each with 65534 hosts 255.255.128.0 510 subnets each with 32768 hosts 255.255.192.0 1022 subnets each with 16382 hosts 255.255.255.0 65534 subnets each with 254 hosts

Example : Class A Subnet Address Table IP Address : 10.0.0.0/16 SubnetID all 0s 10.0.0.0 10.0.0.1 10.0.255.254 10.0.255.255 #1 10.1.0.0 10.1.0.1 10.1.255.254 10.1.255.255 #2 10.2.0.0 10.2.0.1 10.2.255.254 10.2.255.255 Network Address Broadcast Address #254 10.254.0.0 10.254.0.1 10.254.255.254 10.254.255.255 10.255.0.0 10.255.0.1 10.255.255.254 10.255.255.255 SubnetID all 1s

Class A Subnet with router 10.0.0.0/16 254 subnets each with 65534 hosts 10.1.0.0 10.1.0.1 to 10.1.255.254 #1 10.2.0.0 10.2.0.1 to 10.2.255.254 #2 #3 10.3.0.0 10.3.0.1 to 10.3.255.254 #254 10.255.0.0 10.255.0.1 to 10.255.255.254

Subnet Class B Example subnet mask Interpretation 255.255.0.0 1 network with 65534 hosts (default subnet) 255.255.192.0 2 subnets each with 16382 hosts 255.255.252.0 62 subnets each with 1022 hosts 255.255.255.0 254 subnets each with 254 hosts 255.255.255.252 16382 subnets each with 2 hosts

Example : Class B Subnet Address Table IP Address : 176.16.0.0 /24 SubnetID all 0s 172.16.0.0 172.16.0.1 172.16.0.254 172.16.0.255 #1 172.16.1.0 172.16.1.1 172.16.1.254 172.16.1.255 #2 172.16.2.0 172.16.2.1 172.16.2.254 172.16.2.255 Network Address Broadcast Address #254 172.16.254.0 172.16.254.1 176.16.254.254 176.16.254.255 176.16.255.0 176.16.255.1 176.16.255.254 176.16.255.255 SubnetID all 1s

Class B Subnet with router 172.16.1.0 172.16.1.1 to 172.16.1.254 172.16.1.0/24 254 subnets each with 65534 hosts #1 172.16.2.0 172.16.2.1 to 172.16.2.254 #2 #3 172.16.3.0 172.16.3.1 to 172.16.3.254 172.16.254.0 172.16.254.1 to 172.16.254.254 #254

Subnet Class C Example subnet mask Interpretation 255.255.255.0 1 network with 254 hosts (default subnet) 255.255.255.192 2 subnets each with 62 host 255.255.255.224 6 subnets each with 30 hosts 255.255.255.240 14 subnets each with 14 hosts 255.255.255.252 62 subnets each with 2 hosts

Example : Class C Subnet Address Table IP Address : 192.68.0.0 /27 SubnetID all 0s 192.68.0.0 192.68.0.1 192.68.0.30 192.68.0.31 #1 192.68.0.32 192.68.0.33 192.68.0.62 192.68.0.63 #2 192.68.0.64 192.68.0.65 192.68.0.94 192.68.0.95 Network Address Broadcast Address #6 192.68.0.192 192.68.0.193 192.68.0.222 192.68.0.223 192.68.0.224 192.68.0.225 192.68.0.254 192.68.0.255 SubnetID all 1s

Class C Subnet with router 192.68.0.0/27 6 subnets each with 30 hosts 192.68.0.32 192.68.0.33 to 192.68.0.62 #1 192.68.0.64 192.68.0.65 to 192.68.0.94 #2 #3 192.68.0.96 192.68.0.97 to 192.68.0.126 192.68.0.192 192.68.0.193 to 192.68.0.222 #6

Subnet Exercise (1) Given IP address 161.200, find out the following to yield not more than 256 hosts per subnet net mask= ?? start net id =?? end net id=?? #of subnet =??

Subnet Exercise (2) Given IP address 192.150.251, find out the following to yield not more than 32 hosts per subnet net mask= ?? start net id =?? end net id=?? #of subnet =??

Type of Subnetting Static Subnetting Variable Lengh Subnetting all subnets in the subnetted network use the same subnet mask pros: simply to implement, easy to maintain cons: wasted address space (consider a network of 4 hosts with 255.255.255.0 wastes 250 IP) the subnets may use different subnet masks pros: utilize address spaces cons: required well-management Variable Lengh Subnetting

Problem of Static subnetting 192.68.0.32/27 used 20 hosts, waste 10 hosts 192.68.0.64/27 used 20 hosts, waste 10 hosts 192.68.0.96/27 used 25 hosts, waste 5 hosts Inefficient allocation of the address space 192.68.0.128/27 used 25 hosts, waste 5 hosts 192.68.0.192/27 used 10 hosts, waste 20 hosts 192.68.0.224/27 used 10 hosts, waste 20 hosts

Variable-Length Subnetting 192.68.0.32/27 used 20 hosts, waste 10 hosts General Idea of VLSM A small subnet with only a few hosts needs a subnet mask that accommodate only few hosts A subnet with many hosts need a subnet mask to accommodate the large number of hosts 192.68.0.64/27 used 20 hosts, waste 10 hosts 192.68.0.96/27 used 25 hosts, waste 5 hosts 192.68.0.128/27 used 25 hosts, waste 5 hosts 192.68.0.192/28 used 10 hosts, waste 4 hosts 192.68.0.208/28 unused subnet Available 14 hosts 192.68.0.224/28 used 10 hosts, waste 4 hosts 192.68.0.240/28 unused subnet Available 14 hosts

VLSM - An Example three different VLSM of 172.16.0.0 CPC RDI 255.255.255.0 255.255.255.0 255.255.255.252 255.255.255.252 255.255.255.252 point-to-point link CPE 255.255.255.252 255.255.255.192