Subnetworks Lectures
3
Subnetwork From Wikipedia, the free encyclopedia. The word subnetwork (subnet for short) has two related meanings. In the older and more general meaning, it meant one physical network of an internetwork. In the Internet Protocol (IP), a subnetwork (usually known as a subnet) is a division of a classful network. The rest of this article is about the second meaning. Subnetting an IP network allows you to break down what appears (logically) to be a single large network into smaller ones. It was originally introduced before the introduction of classful network number s in IPv4, to allow a single site to have a number of local area networks. Even after the introduction of classful network numbers, it continued to be useful, as it reduced the number of entries in the Internet-wide routing table (by hiding information about all the individual subnets inside a site). As a side benefit, it also resulted in reduced network overhead, by dividing the parts which receive IP broadcasts. Network masks A network mask, also known as a subn et mask, netmask or address mask, is a bitmask used to tell how much of an IP address identi fies the subnetwork the host is on and how much identifies the host. Subnet masks are usually represented in the same representation used for addresses themselves; in IPv4, dotted quad notation (four numbers from zero to 255 separated by periods, e.g. 255.128.0.0) or, less commonly, as an eight-digit hexadecimal number (e.g. FF.80.00.00 = 255.128.0.0). A shorter form, which is known as Classless Inter-Domain Routing (CIDR) notation, gives the network number followed by a slash and the number of 'one' bits in the binary notation of the netmask (i.e. the number of relevant bits in the network number). For example, 192.0.2.96/28 indicates an IP address where the first 28 bits are used as the network address (same as 255.255.255.240). Subnetworking concept IPv4 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. There are three classes of IP address which determine how much is which. Class First bits Start End Mask in dotted decimal A 0 1.0.0. 0 127.0.0 .0 255.0. 0.0 B 10 128.0.0.0 191.255.0.0 255.255.0.0 C 110 192.0.0.0 223.255.255.0 255.255.255.0 Subnetting is the process of allocating bits from the host portion as a network portion. For example, giving the class A network 10.0.0.0 a subnet mask of 255.255.0.0 would break it down into 256 sub-networks (10.0.0.0 to 10.255.0.0). Indicating that the first octet of the IP address shows the network address, the second one shows the subnet number and the last two shows the host part. A bitwise AND operation of the host PDF created with pdfFactory trial version www.pdffactory.com
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Transcript of Subnetworks Lectures
7/18/2019 Subnetworks Lectures
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SubnetworkFrom Wikipedia, the free encyclopedia.
The word subnetwork (subnet for short) has two related meanings. In the older and moregeneral meaning, it meant one physical network of an internetwork. In the Internet
Protocol (IP), a subnetwork (usually known as a subnet) is a division of a classfulnetwork. The rest of this article is about the second meaning.
Subnetting an IP network allows you to break down what appears (logically) to be asingle large network into smaller ones. It was originally introduced before theintroduction of classful network numbers in IPv4, to allow a single site to have a numberof local area networks. Even after the introduction of classful network numbers, itcontinued to be useful, as it reduced the number of entries in the Internet-wide routingtable (by hiding information about all the individual subnets inside a site). As a sidebenefit, it also resulted in reduced network overhead, by dividing the parts which receiveIP broadcasts.
Network masks
A network mask, also known as a subnet mask, netmask or address mask, is a bitmaskused to tell how much of an IP address identifies the subnetwork the host is on and howmuch identifies the host.
Subnet masks are usually represented in the same representation used for addressesthemselves; in IPv4, dotted quad notation (four numbers from zero to 255 separated byperiods, e.g. 255.128.0.0) or, less commonly, as an eight-digit hexadecimal number (e.g.FF.80.00.00 = 255.128.0.0).
A shorter form, which is known as Classless Inter-Domain Routing (CIDR) notation,
gives the network number followed by a slash and the number of 'one' bits in the binarynotation of the netmask (i.e. the number of relevant bits in the network number). Forexample, 192.0.2.96/28 indicates an IP address where the first 28 bits are used as thenetwork address (same as 255.255.255.240).
Subnetworking concept
IPv4 addresses are broken down into three parts, the network part, the subnet part (nowoften considered part of the network part, although originally it was part of the rest part)and the host part. There are three classes of IP address which determine how much iswhich.Class First bits Start End Mask in dotted decimal
A 0 1.0.0.0 127.0.0.0 255.0.0.0B 10 128.0.0.0 191.255.0.0 255.255.0.0C 110 192.0.0.0 223.255.255.0 255.255.255.0
Subnetting is the process of allocating bits from the host portion as a network portion.For example, giving the class A network 10.0.0.0 a subnet mask of 255.255.0.0 wouldbreak it down into 256 sub-networks (10.0.0.0 to 10.255.0.0). Indicating that the firstoctet of the IP address shows the network address, the second one shows the subnetnumber and the last two shows the host part. A bitwise AND operation of the host
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address with the subnet mask extracts the complete subnetwork address (see examplebelow).
Subnet masks are not limited to whole octets, either. For example 255.254.0.0 (or /15) isalso a valid mask. Applied to a class A address this would create 128 subnetworks inintervals of two (1.2.0.1 - 1.3.255.254, 1.4.0.1 - 1.5.255.254, etc).
[edit]
Example
Having the IP address 12.11.10.9 with a class A subnet mask of 255.254.0.0 (the sameas 12.11.10.9/15) says:
Host address 12.11.10.9 Decimal => 00001100 00001011 00001010 00001001 Binary.The first bit is zero, therefore we have a class A network with the network address12.0.0.0 (see classful network).
Subnet mask 255.254.0.0 Decimal => 11111111 11111110 00000000 00000000 Binary.
The subnet mask extends the network address by 7 more bits (254). Thus we have asubnet number of 10 and an extended network address of 12.10.0.0 (decimal) =00001100 00001010 00000000 00000000 (binary) (Bitwise AND of 12.11.10.9 with255.254.0.0).
The remaining host part is 0.1.10.9 Decimal => 00000000 00000001 0000101000001001 Binary.
Determining the number of hosts and subnets on a particular network is quite easy, ifyou know the subnet mask. Say you have the network address 204.4.32.0 with a subnetmask of 255.255.224.0. This network address can also be written as 204.4.32.0/19
Network address 204.4.32.0 Decimal => 11001100.00000100.00100000.00000000Binary
Subnet mask 255.255.224.0 Decimal => 11111111.11111111.11100000.00000000Binary
The subnet mask has 19 bits for the network portion of the address, and 13 bits for thehost part.
23 = 8 possible subnets available according to RFC 1812, otherwise using the old RFC950 standard the number of usable subnets is 6. This is due to RFC 950 not supportingsubnets with either all 1's or all 0's.
213 2 = 8190 possible hosts available to each subnet.
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Class A
0. 0. 0. 0 = 00000000.00000000.00000000.00000000127.255.255.255 = 01111111.11111111.11111111.11111111
0nnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh
Class B128 .0. 0. 0 = 10000000.00000000.00000000.00000000191.255.255.255 = 10111111.11111111.11111111.11111111
10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh
Class C192. 0. 0. 0 = 11000000.00000000.00000000.00000000223.255.255.225 = 11011111.11111111.11111111.11111111
110nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh
Class D224. 0. 0. 0 = 11100000.00000000.00000000.00000000
239.255.255.255 = 11101111.11111111.11111111.111111111110XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX
Class E240. 0. 0. 0 = 11110000.00000000.00000000.00000000255.255.255.255 = 11111111.11111111.11111111.11111111
1111XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX
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