The approximately 4.3 billion IP addresses include not only those that are normally assigned to hosts, but also those that are used for special purposes. In this part, I would like to take a look at such special IP addresses.
IP addresses divided into 5 classes
IP addresses are represented by 32-digit binary numbers, so they can be used as about 4.3 billion unique identifiers. However, some of them include IP addresses that are used for special purposes, and not all of them can be used freely.
First, IP addresses are classified into five groups, A through E, based on a concept called "class" (Fig. 1). Of these five classes, classes D and E are used for special purposes. The former is called a "multicast address" and is used for simultaneous distribution of video, such as streaming, to hosts grouped under specific conditions. When a router that supports multicast receives a packet addressed to a class D IP address, it forwards it to a host belonging to a preset multicast group. Class E IP addresses on the other hand are reserved for future use and cannot be used.
Figure 1 IP address and class
IP Addresses with Special Meaning
The IP addresses introduced so far were special in the sense that they could not be assigned to a host. Here are some IP addresses that are assigned to hosts but have special meanings.
Take a look at Table 1 first. These are the main special IP addresses, but the most familiar one is probably the "private IP address (class C)" in the range of "192.168.0.0 to 192.168.255.255".
Table 1 Special IP addresses (excerpts from RFC 3330)
Private IP addresses are addresses that can be freely assigned to hosts that are not directly connected to the Internet. Originally, if IP addresses were assigned freely, they might be duplicated everywhere, and they would not be able to fulfill their role as identifiers. For this reason, hosts connected to the Internet are assigned addresses called "global IP addresses," which are carefully managed by an organization called ICANN (Internet Corporation for Assigned Names and Numbers).
But there are only about 4.3 billion IP addresses. That was enough when the Internet was invented, but now it falls short. However, it will take time to migrate to IPv6, so we thought of a way to save global IP addresses while staying with IPv4. That is the combination with a private IP address. in particular,
I created a rule calledto reduce the consumption of valuable global IP addresses.
(Continued on next page, "Other Special Addresses")
