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Network Working Group David D. CLARK
Request for Comments: 932 mit, LCS
January 1985
Subnet address assignment scheme
(RFC932 - A Subnetwork Addressing Scheme)
The state of this memo
This document is a proposal for the INTERNET protocol and needs to be discussed. The release of this memo is not restricted.
Summary
Recently, several RFS documents discussed the need for the "subnet" structure in the Internet address assignment scheme.
And submit subnet address assignments and routing strategies. In particular, Jeff Mogul is in RFC917 "Internet Subnet",
An address allocation scheme is described, and some bit of the start address part is used to identify the subnet. The lack of this program
Point is to modify the implementation of the existing host software. Although the changes are very simple, all hosts need to update. (please
See the description of the different methods of this problem in Jeff Mogul RFC917).
This document proposes another subnet address allocation scheme. This program does not need to modify the host soft in most cases
Part. Its disadvantage is that the number of subnets in a network is limited, and all gateways need to be modified.
purpose
In this scenario, the subnet in the network is identified with the network address of the C network. Therefore, we keep C networks
The upper half of the address space, that is, the highest bit is 1 of those addresses. When you want to organize a network into a series
When the subnet, give this network allocated some consecutive reserved Class C network addresses, the top 2 words of these addresses
The section is the same. Thus, different subnets distinguish between the third byte of the address. This program puts a network
The subnet is limited to 256. If you need more subnets, you need two addresses, and this network is
Two networks. The gateway and the host in this network use these addresses like the normal C class address, so the host
The software does not have to change.
For gateways that are not directly connected to the network of the subnet, it is impossible to save these subnets separately.
Routing information. The purpose of the subnet allocation scheme is to let the gateway on this network use the entire network as one.
Overall routing. In this scenario, because the first two bytes of all subnet addresses in a network are the same, far
The gateway can use the first two bytes as the address of a Class B network to store and process. This can be
At the time, the address of the Class C and C network address was called "B 1/2" class address.
In more detail, the gateway can processes the Class C address:
Looking at the high level of the address is 1, if not, the address is a normal C class address to do corresponding processing. in case
The high level is 1, then this address is the subnet address of a network. The test gateway is not even on this network. in case
Yes, this address is processed as a normal C-class address. If the gateway is not connected to this network, then remove the address.
The third byte, treat the remaining two bytes as class B address. Note that this mode Class B address will not conflict with ordinary B address, because the first byte of this address is not a first byte of a valid B address,
But Class C address.
optimization
If a network of a network is more than 256, you need to assign two special Class C address space to become two
Independent network. However, the gateway between the two networks can be appropriately designed, using a synthesis of these two parts
Optimized routing algorithm. It is also possible when there are many gateways.
In some special occasions, if the gateway connected to this network can save a separate C class address.
This is also possible to improve network efficiency.
As mentioned earlier, this program does not have to change the host software. In the case of the following, a small amount of modification will have a very
Big help. Consider a host that is directly connected to the network that is divided into the positive net, it can safeguard this network
The respective routing information of each subnet. For most hosts, it only remembers the road to the host of interest.
By information, the information to remember these subnets will not cause problems. However, if some reasons, hosts are
Remember the routing information of all the addresses, the software's software must learn from the remote gateway to two bytes and three bytes.
The processing algorithm for the site. We do not recommend this method. Under normal circumstances, the host is not necessary to handle C-class
Conversion of the address to class B address.
Disadvantage
The "fold" address of the following other modes can be imagined. For example, 256 B addresses can form a A
Class address. However, since the first 3 bits of this Class A address are constrained, there may be 32 such subnets.
Another more interesting method is to fold the C class address into a class address. Obviously, this is not the most organized address
A good method, but such a combination can make the existence of a large number of subnets.
The most interesting method of "B1 / 2" address is to increase the length of the field of the identification subnet. For example, there are 10 bits for
Mosetting subnet, so that there can be 1024 subnets in each network. When the gateway forms the corresponding address, not only go
After the third byte, you also remove the last two of the class B address. Since the top 3 is constrained, 13 can be given
Subnet use, that is, 8192 subnets. This number is not very big. Therefore, choose the appropriate subnet word
The paragraph is very important.
Danny Cohen recommends that the boundaries of the network, subnets, and host locations can be changed. This real
The current problem is how the gateway should maintain the address of "folding". This RFC suggests that only one "B1 / 2" is implemented in the short term.
The address is the Internet standard.
RFC932 - A SubnetWork Addressing Scheme Subnet Address Assignment Plan
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