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Address Allocation

Before a local network can be connected to the Internet it must be allocated a unique IP address. These addresses are allocated by ISI. The allocation process consists of getting an application form received from ISI. (Send a message to hostmaster@sri-nic.arpa and ask for the template for a connected address). This template is filled out and mailed back to hostmaster. An address is allocated and e-mailed back to you. This can also be done by postal mail (Appendix B).

IP addresses are 32 bits long. It is usually written as four decimal numbers separated by periods (e.g., Each number is the value of an octet of the 32 bits. It was seen from the beginning that some networks might choose to organize themselves as very flat (one net with a lot of nodes) and some might organize hierarchically (many interconnected nets with fewer nodes each and a backbone). To provide for these cases, addresses were differentiated into class A, B, and C networks. This classification had to with the interpretation of the octets. Class A networks have the first octet as a network address and the remaining three as a host address on that network. Class C addresses have three octets of network address and one of host. Class B is split two and two. Therefore, there is an address space for a few large nets, a reasonable number of medium nets and a large number of small nets. The top two bits in the first octet are coded to tell the address format. All of the class A nets have been allocated. So one has to choose between Class B and Class C when placing an order. (There are also class D (Multicast) and E (Experimental) formats. Multicast addresses will likely come into greater use in the near future, but are not frequently used now).

In the past sites requiring multiple network addresses requested multiple discrete addresses (usually Class C). This was done because much of the software available (notably 4.2BSD) could not deal with subnetted addresses. Information on how to reach a particular network (routing information) must be stored in Internet gateways and packet switches. Some of these nodes have a limited capability to store and exchange routing information (limited to about 300 networks). Therefore, it is suggested that any campus announce (make known to the Internet) no more than two discrete network numbers.

If a campus expects to be constrained by this, it should consider subnetting. Subnetting (RFC-932) allows one to announce one address to the Internet and use a set of addresses on the campus. Basically, one defines a mask which allows the network to differentiate between the network portion and host portion of the address. By using a different mask on the Internet and the campus, the address can be interpreted in multiple ways. For example, if a campus requires two networks internally and has the 32,000 addresses beginning 128.174.X.X (a Class B address) allocated to it, the campus could allocate 128.174.5.X to one part of campus and 128.174.10.X to another. By advertising 128.174 to the Internet with a subnet mask of FF.FF.00.00, the Internet would treat these two addresses as one. Within the campus a mask of FF.FF.FF.00 would be used, allowing the campus to treat the addresses as separate entities. (In reality you don't pass the subnet mask of FF.FF.00.00 to the Internet, the octet meaning is implicit in its being a class B address).

A word of warning is necessary. Not all systems know how to do subnetting. Some 4.2BSD systems require additional software. 4.3BSD systems subnet as released. Other devices and operating systems vary in the problems they have dealing with subnets. Frequently these machines can be used as a leaf on a network but not as a gateway within the subnetted portion of the network. As time passes and more systems become 4.3BSD based, these problems should disappear.

There has been some confusion in the past over the format of an IP broadcast address. Some machines used an address of all zeros to mean broadcast and some all ones. This was confusing when machines of both type were connected to the same network. The broadcast address of all ones has been adopted to end the grief. Some systems (e.g. 4.2 BSD) allow one to choose the format of the broadcast address. If a system does allow this choice, care should be taken that the all ones format is chosen. (This is explained in RFC-1009 and RFC-1010).

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National Technical University of Athens