How Does This Internet Thing Work? Mystery Cache

Hosts and networks

IP addressing is based on the concept of hosts and networks. A host is essentially anything on the network that is capable of receiving and transmitting IP packets on the network, such as a workstation or a router. It is not to be confused with a server: servers and client workstations are all IP hosts.

The hosts are connected together by one or more networks. The IP address of any host consists of its network address plus its own host address on the network. IP addressing, unlike, say, IPX addressing, uses one address containing both network and host address. How much of the address is used for the network portion and how much for the host portion varies from network to network.

IP addressing

An IP address is 32 bits wide, and as discussed, it is composed of two parts: the network number, and the host number. By convention, it is expressed as four decimal numbers separated by periods, such as "200.1.2.3" representing the decimal value of each of the four bytes. This is known as dotted decimal notation. Each decimal number is known as an octet and may range in value from 0 to 255. Valid addresses thus range from 0.0.0.0 to 255.255.255.255, a total of 4,294,967,296 addresses. The first few bits of the address indicate the Class that the address belongs to:

Table 1

The bits are labeled in network order, so that the first bit is bit 0 and the last is bit 31, reading from left to right. Class D addresses are multicast, and Class E are reserved. For normal Internet addressing only classes A, B, and C are used. The range of network numbers and host numbers may then be derived:

Table 2

Any address whos first octet is 127 is considered a loop back address and should never be used for addressing outside the host. A host number of all binary 0s represents the network address. A host number of all binary 1s indicates a directed broadcast over the specific network. For example, 200.1.2.255 would indicate a broadcast over the 200.1.2 network. All the reserved bits and reserved addresses severely reduce the available IP addresses from the 4.3 billion theoretical maximum.

So how many computers can there be on the Internet using this addressing scheme? We know from above that the theoretical maximum number of hosts is 4,294,967,296. Given the restrictions imposed by the design of the address space the actual theoretical maximum is something less. How would we figure out what that is? Lets look at it one Class at a time. Let's look at the Class B address space.

From the information presented above in Table 1 we can see that the Class B address space utilizes 16 bits for its network number. We also see that two bits of the network number are consumed by the prefix giving us an effective 14-bit network number. With this information we can calculate how many networks exist in the Class B address space by raising 2 to the 14^th power. This calculation reveals that there are 16,384 networks in the Class B address space.

Also from Table 1 we see that the host number is 16 bits as well. We can easily calculate the number of hosts available in each Class B network by raising 2 to the 16^th power. That calculation tells us that there are 65,536 hosts in each network. This is not quite correct, however. Recall from above that two numbers are reserved in each network for the network address and the broadcast address.

So, for Class B we have 16,384 networks, each containing 65,534 hosts. Simple mutiplication yields 1,073,709,056 possible hosts in the Class B address space.

What is the total maximum number of hosts that can possibly exist in the Class A, B, and C networks?

Where is the cache?

The answer to the problem is a 10-digit number. Label the digits from left to right A,BCD,EFG,HJK. Calculate the value of L by subtracting K from D.

The cache is at N 47 AG.FLD W 122 GK.FJE.

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