Ramjee Prasad Reg no-10804900 Roll-RE6701B53 B.TECH (LEET) ECE Abstract:In my abstract i am going to discuss about the definition of subnetting, binary subnet masking, restrictive subnet masks and examples, network address and logical address, IPV4 classes, IPV6 classes, subnet and host count.

in quad-dotted decimal representation, e.g., 255.255.255.0 is the subnet mask for the 192.168.1.0 network with a 24-bit routing prefix (192.168.1.0/24). For IPv6 networks, routing prefixes are always expressed in the standardized CIDR notation consisting of the network address and the mask length, e.g., 2001:db8::/32. All hosts within a subnet can be reached in one "hop" (time to live = 1), implying that all hosts in a subnet are connected to the same link. A typical subnet is a physical network served by one router, for instance an Ethernet network (consisting of one or several Ethernet segments or local area networks, interconnected by network switches and network bridges) or a Virtual Local Area Network (VLAN). However, subnetting allows the network to be logically divided regardless of the physical layout of a network, since it is possible to divide a physical network into several subnets by configuring different host computers to use different routers. While improving network performance, subnetting increases routing complexity, since each locally connected subnet is typically represented by one row in the routing tables in each connected router. However, with a clever design of the network, routes to collections of more distant subnets within the branches of a tree-hierarchy can be aggregated by single routes. Existing subnetting functionality in routers made the introduction of Classless InterDomain Routing seamless. A graphic representation of relationships and source of the various variables representing a chunk of /24 subnets.

SubnettingA subnetwork, or subnet, describes networked computers and devices that have a common, designated IP address routing prefix. Subnetting is used to break the network into smaller more efficient subnets to prevent excessive rates of Ethernet packet collision in a large network. Such subnets can be arranged hierarchically, with the organization's network address space (see also Autonomous System) partitioned into a tree-like structure. Routers are used to manage traffic and constitute borders between subnets. Subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used to connect IP networks to minimize the amount of traffic each segment must receive. A routing prefix is the sequence of leading bits of an IP address that precede the portion of the address used as host identifier. In IPv4 networks, the routing prefix is often expressed as a "subnet mask", which is a bit mask covering the number of bits used in the prefix. An IPv4 subnet mask is frequently expressed

Network address and logical addressThe term network address sometimes refers to logical address, i.e. network layer address such as the IP address, and sometimes to the first address (the base address) of a classful address range to an organization. Computers and devices that are part of an internetworking network such as the Internet each have a logical address. The network address is unique to each device and can either be dynamically or statically configured. An address allows a device to communicate with other devices connected to a network. The most common network addressing scheme is IPv4. An IPv4 address consists of a 32 bit

address written, for human readability, into 4 octets and a subnet mask of like size and notation. In order to facilitate the routing process the address is divided into two pieces: The network prefix (some contiguous range of higher-order bits) that is significant for routing decisions at that particular topological point. The network host (the remaining bits) that specify a particular device in the network.

Class C 255.255.255.0 11111111.11111111.11111111.00000000

Binary subnet masksWhile subnet masks are often represented in dotdecimal form, their use becomes clearer in binary. Looking at a network address and a subnet mask in binary, a device can determine which part of the address is the network address and which part is the host address. To do this, it performs a bitwise "AND" operation.

This works much like a postal address in that network prefix would represent the city and the network host would represent the address of a specific house on that street. The subnet mask (e.g. 255.255.192.0 to specify the top 18 bits; in binary: 11111111.11111111.11000000.00000000) or CIDR suffix address (e.g. /18) is used in conjunction with the network address to determine how many higherorder bits are used for the network prefix. For instance, the following are equivalent: 192.168.0.0 with netmask 255.255.0.0 192.168.0.0/16

ExampleDotdecimal Address

Binary

IP 192.168.5.1 11000000.10101000.00000101.0 addres 0 0001010 s

Subnet MaskingApplying a subnet mask to an IP address allows you to identify the network and node parts of the address. The network bits are represented by the 1s in the mask, and the node bits are represented by the 0s. Performing a bitwise logical AND operation between the IP address and the subnet mask results in the Network Address or Number. For example, using our test IP address and the default Class B subnet mask, we get: 10001100.10110011.11110000.11001000 140.179.240.200 Class B IP Address 11111111.11111111.00000000.00000000 255.255.000.000 Default Class B Subnet Mask -------------------------------------------------------10001100.10110011.00000000.00000000 140.179.000.000 Network Address Default subnet masks: Class A 255.0.0.0 11111111.00000000.00000000.00000000 Class B 255.255.0.0 11111111.11111111.00000000.00000000 Subnet 255.255.25 11111111.11111111.11111111.0 Mask 5.0 0000000

Netwo rk 11000000.10101000.00000101.0 192.168.5.0 Portio 0000000 n

Host Portio 0.0.0.10 n

00000000.00000000.00000000.0 0001010

Subnet masks consist of 32 bits, usually a block of ones (1) followed by a block of 0s. The last block of zeros (0) designate that part as being the host identifier. This allows a classful network to be broken down into subnets. A classful network is a network that has a subnet mask of 255.0.0.0, 255.255.0.0 or 255.255.255.0.

More Restrictive Subnet MasksAdditional bits can be added to the default subnet mask for a given Class to further subnet, or break down, a network. When a bitwise logical AND operation is performed between the subnet mask and IP address, the result defines the Subnet Address (also called the Network Address or Network Number). There are some restrictions on the subnet address. Node addresses of all "0"s and all "1"s are reserved for specifying the local network (when a host does not know its network address) and all hosts on the network (broadcast address), respectively. This also applies to subnets. A subnet address cannot be all "0"s or all "1"s. This also implies that a 1 bit subnet mask is not allowed. This restriction is required because older standards enforced this restriction. Recent standards that allow use of these subnets have superseded these standards, but many "legacy" devices do not support the newer standards. If you are operating in a controlled environment, such as a lab, you can safely use these restricted subnets. To calculate the number of subnets or nodes, use the formula (2n-2) where n = number of bits in either field, and 2n represents 2 raised to the nth power. Multiplying the number of subnets by the number of nodes available per subnet gives you the total number of nodes available for your class and subnet mask. Also, note that although subnet masks with noncontiguous mask bits are allowed, they are not recommended.

between the Subnet address and the Broadcast address. This gives a total of 49,140 nodes for the entire class B address subnetted this way. Notice that this is less than the 65,534 nodes an unsubnetted class B address would have. You can calculate the Subnet Address by performing a bitwise logical AND operation between the IP address and the subnet mask, then setting all the host bits to 0s. Similarly, you can calculate the Broadcast Address for a subnet by performing the same logical AND between the IP address and the subnet mask, then setting all the host bits to 1s. That is how these numbers are derived in the example above. Subnetting always reduces the number of possible nodes for a given network. There are complete subnet tables available here for Class A, Class B and Class C. These tables list all the possible subnet masks for each class, along with calculations of the number of networks, nodes and total hosts for each subnet. An Example Here is another, more detailed, example. Say you are assigned a Class C network number of 200.133.175.0 (apologies to anyone who may actually own this domain address). You want to utilize this network across multiple small groups within an organization. You can do this by subnetting that network with a subnet address. We will break this network into 14 subnets of 14 nodes each. This will limit us to 196 nodes on the network instead of the 254 we would have without subnetting, but gives us the advantages of traffic isolation and security. To accomplish this, we need to use a subnet mask 4 bits long. Recall that the default Class C subnet mask is 255.255.255.0 (11111111.11111111.11111111.00000000 binary) Extending this by 4 bits yields a mask of 255.255.255.240 (11111111.11111111.11111111.11110000 binary) This gives us 16 possible network numbers, 2 of which cannot be used: Subnet Network bits Number Node Broadcast Addresses Address

Example:10001100.10110011.11011100.11001000 140.179.220.200 IP Address 11111111.11111111.11100000.00000000 255.255.224.000 Subnet Mask -------------------------------------------------------10001100.10110011.11000000.00000000 140.179.192.000 Subnet Address 10001100.10110011.11011111.11111111 140.179.223.255 Broadcast Address In this example a 3 bit subnet mask was used. There are 6 (23-2) subnets available with this size mask (remember that subnets with all 0's and all 1's are not allowed). Each subnet has 8190 (213-2) nodes. Each subnet can have nodes assigned to any address

0000 0001

200.133.175.0 200.133.175.16

Reserved

None

.17 thru . 200.133.175.31 30 .33 thru . 200.133.175.47 46 .49 thru . 200.133.175.63 62 .65 thru . 200.133.175.79 78 .81 thru . 200.133.175.95 94

IPv4 classesIPv4 addresses are broken down into three parts: the network part, the subnet part (now often considered part of the network part, although originally it was part of the rest part), and the host part. Even though classful networks are obsolete, both classful and classless networks are shown in the following table.

0010

200.133.175.32

0011

200.133.175.48

0100

200.133.175.64

0101

200.133.175.80

0110

.97 thru . 200.133.175.96 200.133.175.111 110 200.133.175.11 .113 thru . 200.133.175.127 2 126 200.133.175.12 .129 thru . 200.133.175.143 8 142 200.133.175.14 .145 thru . 200.133.175.159 4 158 200.133.175.16 .161 thru . 200.133.175.175 0 174 200.133.175.17 .177 thru . 200.133.175.191 6 190 200.133.175.19 .193 thru . 200.133.175.207 2 206 200.133.175.20 .209 thru . 200.133.175.223 8 222 200.133.175.22 .225 thru . 200.133.175.239 4 238 200.133.175.24 Reserved 0 None

Class

Leadin Start g bits

End

Default Subnet Mask dotted decimal

in

0111

1000

A (CID 0 R /8)

0.0.0.0

127.255.255.2 255.0.0.0 55

1001

1010

B (CID 10 R / 16)

128.0.0. 191.255.255.2 255.255.0.0 0 55

1011

C (CID 110 R / 24)

192.0.0. 223.255.255.2 255.255.255. 0 55 0

1100

1101

D

1110

224.0.0. 239.255.255.2 0 55

1110

E

1111

240.0.0. 255.255.255.2 0 54

1111

While the 127.0.0.0/8 network is in the Class A area, it is designated for loopback and cannot be assigned to a network.

Class D multicasting Class E reserved Subnetting is the process of allocating bits from the host portion as a network portion. The above example shows the bitwise "AND" process being performed on a classful network. The following example shows bits being borrowed to turn a classful network into a subnet

subnetworks. Each bit can take the value 1 or 0, giving 4 possible subnets (22 = 4) Network Network (binary) Broadcast address

192.168.5.0 11000000.10101000.0000010 192.168.5. /26 1.00000000 63 192.168.5.6 11000000.10101000.0000010 192.168.5. 4/26 1.01000000 127 192.168.5.1 11000000.10101000.0000010 192.168.5. 28/26 1.10000000 191 192.168.5.1 11000000.10101000.0000010 192.168.5. 92/26 1.11000000 255

ExampleDot-decimal Binary Address

IP 192.168.5.13 11000000.10101000.00000101. addres 0 10000010 s

According to the RFC 950 standard the subnet values consisting of all zeros and all ones are reserved, reducing the number of available subnets by 2. However due to the inefficiencies introduced by this convention it is no longer used on the public Internet, and is only relevant when dealing with legacy equipment that does not understand CIDR. The only reason not to use the all-zeroes subnet is that it is ambiguous when the exact suffix length is not available. All CIDR-compliant routing protocols transmit both length and suffix. See RFC 1878 for a subnetting table with extensive examples. The remaining bits after the subnet are used for addressing hosts within the subnet. In the above example the subnet mask consists of 26 bits, leaving 6 bits for the address (32 26). This allows for 64 possible combinations (26), however the all zeros value and all ones value are reserved for the network ID and broadcast address respectively, leaving 62 addresses. In general the number of available hosts on a subnet can be calculated using the formula 2n 2, where n is the number of bits used for the host portion of the address. RFC 3021 specifies an exception to this rule when dealing with 31 bit subnet masks (i.e. 1 host bit). According to the above rule a 31 bit mask would allow for 21 2 = 0 hosts. The RFC makes allowances in this case for certain types of networks (point-to-point) to disregard the network and broadcast address, allowing two host addresses to be allocated. Possible subnets for a /24 suffix (traditional Class C):

Subne 255.255.255. 11111111.11111111.11111111. t 192 11000000 Mask

Netwo rk 192.168.5.12 11000000.10101000.00000101. Portio 8 10000000 n

In this example two bits were borrowed from the original host portion. This is beneficial because it allows this network to be split into four smaller networks. A /24 suffix (Class C block) allows 254 hosts; split into four parts, the prefix is /26, each has 62 hosts.

Subnets and host countIt is possible to determine the number of hosts and subnetworks available for any subnet mask. In the above example two bits were borrowed to create

CIDR Available Total Available notatio Network Mask Hosts per usable Networks n network hosts /24 /25 /26 /27 /28 /29 /30 /31 255.255.255.0 1 254 126 62 30 14 6 2 2* 254 252 248 240 224 192 128 256 255.255.255.12 2 8 255.255.255.19 4 2 255.255.255.22 8 4 255.255.255.24 16 0 255.255.255.24 32 8 255.255.255.25 64 2 255.255.255.25 128 4

addresses. The "classful" system of allocating IP addresses can be very wasteful; anyone who could reasonably show a need for more that 254 host addresses was given a Class B address block of 65533 host addresses. Even more wasteful were companies and organizations that were allocated Class A address blocks, which contain over 16 Million host addresses! Only a tiny percentage of the allocated Class A and Class B address space has ever been actually assigned to a host computer on the Internet. People realized that addresses could be conserved if the class system was eliminated. By accurately allocating only the amount of address space that was actually needed, the address space crisis could be avoided for many years. This was first proposed in 1992 as a scheme called Supernetting. Under supernetting, the classful subnet masks are extended so that a network address and subnet mask could, for example, specify multiple Class C subnets with one address. For example, If I needed about 1000 addresses, I could supernet 4 Class C networks together: 192.60.128.0 (11000000.00111100.10000000.00000000) Class C subnet address 192.60.129.0 (11000000.00111100.10000001.00000000) Class C subnet address 192.60.130.0 (11000000.00111100.10000010.00000000) Class C subnet address 192.60.131.0 (11000000.00111100.10000011.00000000) Class C subnet address -------------------------------------------------------192.60.128.0 (11000000.00111100.10000000.00000000) Supernetted Subnet address 255.255.252.0 (11111111.11111111.11111100.00000000) Subnet Mask 192.60.131.255 (11000000.00111100.10000011.11111111) Broadcast address In this example, the subnet 192.60.128.0 includes all the addresses from 192.60.128.0 to 192.60.131.255. As you can see in the binary representation of the subnet mask, the Network portion of the address is 22 bits long, and the host portion is 10 bits long.

* only applicable on point-to-point links

Subnetting in IPv6 networksThe primary reason for subnetting in IPv4 was to improve efficiency in the utilization of the relatively small address space available, particularly to enterprises. Subnetting is also used in IPv6 networks. However, in IPv6 the address space available even to end-users is so large that address space restrictions no longer exist. The recommended allocation for a site is an address space comprising 80 address bits (prefix / 48), but may be as small as a prefix /56 allocation (72 bits) for a residential customer network.[1] This provides 65,536 subnets for a site, and a minimum of 256 subnets for the residential size. An IPv6 subnet always has a /64 prefix which provides 64 bits for the host portion of an address. Although it is technically possible to use smaller subnets, they are impractical for local area networks because stateless address autoconfiguration of network interfaces (RFC 4862) requires a /64 address. Subnetting, based on the concepts of Classless Inter-Domain Routing is however used in the routing between networks both locally and globally. Now that you understand "classful" IP Subnetting principals, you can forget them ;). The reason is CIDR -- Classless InterDomain Routing. CIDR was invented several years ago to keep the internet from running out of IP

Under CIDR, the subnet mask notation is reduced to a simplified shorthand. Instead of spelling out the bits of the subnet mask, it is simply listed as the number of 1s bits that start the mask. In the above example, instead of writing the address and subnet mask as 192.60.128.0, Subnet Mask 255.255.252.0 the network address would be written simply as: 192.60.128.0/22 which indicates starting address of the network, and number of 1s bits (22) in the network portion of the address. If you look at the subnet mask in binary (11111111.11111111.11111100.00000000), you can easily see how this notation works. The use of a CIDR notated address is the same as for a Classful address. Classful addresses can easily be written in CIDR notation (Class A = /8, Class B = / 16, and Class C = /24) It is currently almost impossible for an individual or company to be allocated their own IP address blocks. You will simply be told to get them from your ISP. The reason for this is the ever-growing size of the internet routing table. Just 10 years ago, there were less than 5000 network routes in the entire Internet. Today, there are over 100,000. Using CIDR, the biggest ISPs are allocated large chunks of address space (usually with a subnet mask of /19 or even smaller); the ISP's customers (often other, smaller ISPs) are then allocated networks from the big ISP's pool. That way, all the big ISP's customers (and their customers, and so on) are accessible via 1 network route on the Internet. But I digress. It is expected that CIDR will keep the Internet happily in IP addresses for the next few years at least. After that, IPv6, with 128 bit addresses, will be needed. Under IPv6, even sloppy address allocation would comfortably allow a billion unique IP addresses for every person on earth! The complete and gory details of CIDR are documented in RFC1519, which was released in September of 1993.

# bits

2

255.192.0.0

/10

2

419430 8388604 2 209715 12582900 0 104857 14680036 4 524286 15728580 262142 16252804 131070 16514820 65534 32766 16382 8190 4094 2046 1022 510 254 16645636 16710660 16742404 16756740 16760836 16756740 16742404 16710660 16645636 16514820

3

255.224.0.0

/11

6

4 5 6 7 8 9 10 11

255.240.0.0 255.248.0.0 255.252.0.0 255.254.0.0 255.255.0.0

/12 /13 /14 /15 /16

14 30 62 126 254 510 1022 2046 4094 8190 16382 32766 65534

255.255.128.0 /17 255.255.192.0 /18 255.255.224.0 /19 /20

255.255.240.0 12 13 14 15 16 17

255.255.248.0 /21 255.255.252.0 /22 255.255.254.0 /23 255.255.255.0 /24 255.255.255.1 /25 28 255.255.255.1 /26 92 255.255.255.2 /27 24

131070 126

18 Subnet Mask CID # # Hosts Nets R Subnets Hosts * 19

262142 62

16252804

524286 30

15728580

20

255.255.255.2 /28 40 255.255.255.2 /29 48 255.255.255.2 /30 52

104857 14 4 209715 6 0 419430 2 2

14680036

21

12582900

22

8388604

Example routing subnet concept

scenario

based

on

Suppose a home network consists of computers named Foo and Bar, connected to a router, and then via a cable modem to the Internet. The home network is configured as a subnet: address 17.76.99.1 is assigned to Foo, address 17.76.99.2 to Bar, and address 17.76.99.100 to the router. The subnet has been configured so that the first three octets of its members' addresses are all the same subnet id, 17.76.99, and this fact is expressed by the subnet mask 255.255.255.0 (binary 11111111 11111111 11111111 00000000) configured in the router. When Foo sends data to example.com at 208.77.188.166, the router performs a logical AND of the destination example.com address with the subnet mask. It also performs a logical AND of the origin address (17.76.99.1) and recognizes that these two results are different, and therefore sends the data over the Internet, via the subnet's default gateway. When Foo sends data to Bar, however, it determines that the results of the two AND operations are the same, therefore the destination lies within the subnet and the default gateway is not required. The data is transmitted directly from Foo to Bar within the home network.