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Subwork
The word subwork (sub for short) has two related meanings. In the older and more general meaning, it meant one physical work of an interwork. In the Inter Protocol (IP), a subwork (usually known as a sub) is a division of a classful work. The rest of this article is about the second meaning.
Subting an IP work allows you to break down what appears (logically) to be a single large work into smaller ones. It was originally introduced before the introduction of classful work numbers in IPv4, to allow a single site to he a number of local area works. Even after the introduction of classful work numbers, it continued to be useful, as it reduced the number of entries in the Inter-wide routing table (by hiding information about all the individual subs inside a site). As a side benefit, it also resulted in reduced work overhead, by dividing the parts which receive IP broadcasts.
work masks
A work mask, also known as a sub mask, mask or address mask, is a bitmask used to tell how much of an IP address identifies the subwork the host is on and how much identifies the host.
Sub masks are usually represented in the same representation used for addresses themselves; in IPv4, dotted decimal notation - four numbers from zero to 255 separated by periods, e.g. 255.128.0.0.
But in sub masks only some of the numbers are allowed: 0,128,192,224,240,248,252,254,255
Less monly, it can be represented 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 work number followed by a slash and the number of 'one' bits in the binary notation of the mask (i.e. the number of relevant bits in the work number). For example, 192.0.2.96/28 indicates an IP address where the first 28 bits are used as the work address (same as 255.255.255.240).
Subworking concept
IPv4 addresses are broken down into three parts, the work part, the sub part (now often considered part of the work 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 Default Sub Mask in dotted decimal CIDR notation
A 0 1.0.0.0 126.0.0.0 255.0.0.0 /8
B 10 128.0.0.0 191.255.0.0 255.255.0.0 /16
C 110 192.0.0.0 223.255.255.0 255.255.255.0 /24
D 1110 224.0.0.0 239.255.255.0
E 1111 240.0.0.0 255.255.255.0
The 127.0.0.0 work ID is left out because it is designated for loopback and cannot be assigned to a work
Class D multicasting
Class E reserve
d
Subting is the process of allocating bits from the host portion as a work portion. For example, giving the class A work 10.0.0.0 a sub mask of 255.255.0.0 would break it down into 256 sub-works (10.0.0.0 to 10.255.0.0). Indicating that the first octet of the IP address shows the work address, the second one shows the sub number and the last two show the host part. A bitwise AND operation of the host address with the sub mask extracts the plete subwork address (see example below).
Sub masks are not limited to whole octets, either. For example 255.254.0.0 (or /15) is also a valid mask. Applied to a class A address this would create 128 subworks in intervals of two (1.2.0.1 - 1.3.255.254, 1.4.0.1 - 1.5.255.254, etc).
Example
Hing the IP address 12.11.10.9 with a sub mask of 255.254.0.0 (the same as 12.11.10.9/15) says:
Host address 12.11.10.9 Decimal =>00001100 00001011 00001010 00001001 Binary. The most significant bit is zero, therefore we he a class A work with the work address 12.0.0.0 (see classful work).
Sub mask 255.254.0.0 Decimal =>11111111 11111110 00000000 00000000 Binary. The sub mask extends the work address by 7 more bits (254). Thus we he a sub number of 10 and an extended work address of 12.10.0.0 (decimal) = 00001100 00001010 00000000 00000000 (binary) (Bitwise AND of 12.11.10.9 with 255.254.0.0).
The remaining host part is 0.1.10.9 Decimal =>00000000 00000001 00001010 00001001 Binary.
Determining the number of hosts and subs on a particular work is quite easy, if you know the sub mask. Say you he the work address 204.4.32.0 with a sub mask of 255.255.224.0. This work address can also be written as 204.4.32.0/19
work address 204.4.32.0 Decimal =>11001100.00000100.00100000.00000000 Binary
Sub mask 255.255.224.0 Decimal =>11111111.11111111.11100000.00000000 Binary
The sub mask has 19 bits for the work portion of the address, and 13 bits for the host part.
23 = 8 possible subs ailable according to RFC 1812, otherwise using the old RFC 950 standard the number of usable subs is 6. This is due to RFC 950 not supporting subs with either all 1's or all 0's.
213 2 = 8190 possible hosts ailable to each sub.
Nonstandard subs
Sub zero
Technically illegal, but still useable is the extreme first subwork. For example, sub 1.0.0.0 with a sub address of 255.255.0.0. The problem with this sub is that the unicast address for the sub is the same as the unicast address for the entire class A work.
Interleed subs
Longest prefix match
IPv4 subting reference
Classless Inter-Domain Routing
Classful work
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