What is the maximum number of host IP addresses that can exist in a class B network group of answer choices?

The length of the physical address varies according to the networking system, but Ethernet and several others use 48 bits. For communication to occur, two addresses are required: one each for the sending and receiving devices. The IEEE assigns a 24-bit organization unique identifier (OUI) so that organizations can assign the remaining 24 bits to suit their unique needs. Two of the 24 bits assigned as an OUI are control bits. The IEEE Ethernet and allied standards use another address for link service access points (LSAPs), which provide services to Layer 3 protocols.

IP Addresses

TCP/IP within the IPv4 format uses a 32-bit address to identify a machine on a network and the network to which it is attached. IP addresses identify a machine’s connection to the network, not the machine itself. The IP address is the set of numbers that many people see on their workstations, such as 127.40.8.72, which uniquely identifies the device. When such a device is connected to the Internet, as opposed to a closed enterprise, it is at the bottom of a global hierarchy for address assignments. End users “rent” an IP address from their Internet Service Provider (ISP), who receives address assignments from a global network of authoritative registries, whose protocol-related operations are coordinated by IANA. Registry organizations can be a Local Internet Registry (LIR), Regional Internet Registry (RIR), or National Internet Registry (NIR). The list of current registries and their areas of coverage is as follows:

• AfriNIC. Africa region.

• APNIC. Asia/Pacific region.

• ARIN. North America region.

• LACNIC. Latin America and certain Caribbean islands.

• RIPE NCC. Europe, Middle East, and Central Asia.

Of the, two available IP protocol versions—IPv4 and IPv6—IPv4 is by far the most widely used today. It was originally organized into classes:

• Class A (0.0.0.0 to 127.255.255.255) for general use. Class A addresses are for large networks; they use 8 bits for the network ID and 24 bits for the host ID.

• Class B (128.0.0.0 to 191.255.255.255) for general use. Class B addresses are for intermediate networks; they use 16-bit host addresses and 16-bit network addresses.

• Class C (192.0.0.0 to 223.255.255.255) for general use. Class C addresses have only 8 bits for the host address, limiting the number of devices to 256. There are 24 bits for the network address.

• Class D (224.0.0.0 to 239.255.255.255) multicast. Class D is for multicast purposes only; the manner of operation is that each multicast address represents a particular group of hosts. IANA assigns permanent addresses and allocates transient addresses through the network of registries.

• Class E (240.0.0.0 to 255.255.255.255) reserved. Class E addresses have historically been reserved for use by the IETF for experimental purposes, but IANA is currently in the process of changing the designation to private use. At the time of writing, it is unclear what private use means in this context, but it is likely that this is a stopgap measure to avoid running out of addresses while the world waits for IPv6.

Certain blocks of addresses within the available spaces are reserved for private Internets. For example, the Class C range (192.168.0.0 to 192.168.255.255) is available and is what many ISP customers see on their computers in their home network.

Classes A, B, and C are most germane to this discussion, particularly as a foundation for understanding Classless Inter-Domain Routing (CIDR), which is discussed at the end of this section. Readers can see that the classful addressing scheme that has served the Internet so well in past decades is virtually slipping away without notice. It is now officially considered as having a “historic” status.

The term classful addressing comes from the fact that a specific number of bits assign an address to a class, and there are different combinations of possible networks and hosts according to each one. The design accommodates the unique networking requirements of organizations by offering options that match their own distributed computing environment. For example, a national sales force with small operations in 1,000 cities needs a lot of network addresses, but few host addresses. That is how it connects teams of only five or six employees to the rest of the company. Centralized business operations, on the other hand, require the opposite—a lot of host addresses and few network addresses. Table 1-7 summarizes classful network addresses for general-purpose classes.

Table 1-7 Classful Network Addressing

Host IDs with all 0s and all 1s cannot be assigned, which reduces the number of possible hosts by two.

Class A network ID numbers 0 and 127 are reserved, so 2 bits are subtracted in calculations.

Because even centralized operations, where most of the company’s workforce is in the same city, might need a campus network, classful addressing can subdivide a single network into several smaller ones, called subnetworks. Subnetting is accomplished by using subnet masks to change the meaning of an IP address. The subnet mask defines the network and host bits in an associated address and is one way to tell, at a glance, which class is in use. Table 1-8 shows the default masks in both dotted-decimal form and their full binary equivalents. It is customary to use a single zero in the dotted-decimal form to represent eight zeros in an octet.

Table 1-8 Subnet Mask translation

A visual inspection of the masks shown here, along with the total network ID bits in Table 1-7, reveals that bit positions populated by ones align with the network ID. The reverse of that is true and is shown in the number of possible hosts. What is not implicit in the visual part of the scheme is the fact that changes to a subnet mask can increase/decrease the number of hosts, but not the possible number of networks.

The following example uses a Class B subnet mask, where

• n = a decimal position in the network octet

• x = a decimal position in the host octet

Modifications to the mask affect the address as follows:

The network and host octets do not change because this is still a Class B address according to the old classful addressing system. The change must be represented differently:

, Address with new subnet mask: , ,

This form of notation is used in, among other documents, RFC 1812,6 “Requirements for IP Version 4 Routers,” where the rules are laid out for the use of this historical scheme in a CIDR environment. CIDR addressing uses the length/prefix notation for addresses where the prefix represented the number of bits in a subnet mask, but now, it is part of the official convention for addressing. A CIDR address is described as

IP address = ,

In router configurations = n.n.0.0/16

This CIDR naming convention looks exactly like what the legacy Class B mask would be if it were written as such, but it is not a Class B address. Subnetting allows users to get more out of their assigned address space within their own network. Devices with Internet connectivity need to use only those addresses that are in the range and assigned by their local registry.

The lengths of each section of the IP address were carefully chosen to provide maximum flexibility in assigning both network and local addresses. The total length is fixed at 32 bits and is divided into four octets according to the notation used to type the address on a keyboard or write it on paper. To put that description in context, here is a basic example of how an IP address translates from four octets—as people see them—to the 1s and 0s that machines can read. This example uses a common internal IP address.

An IP address written as four octets looks like this:

192.168.1.101

Figure 1-7 shows a way to convert this IP address without a calculator or conversion chart.

Figure 1-7

Quick conversion of an IP address from octets to bits

To use this shortcut by hand, write the address’ decimal version on paper and leave room in between each for the values underneath. Because each decimal value represents an octet, 8-bit positions are populated in the next line, as Figure 1-7 shows. The last step is to add whichever numbers from the 8-bit positions equal the decimal value; fill in 1s underneath those values and 0s underneath those that were not used. The result is a 32-bit binary representation of the IP address.

From the IP address, a network can determine if the data will be sent out through a gateway. If the network address is the same as the current address (routing to a local network device, called a direct host), the gateway is avoided, but all other network addresses are routed to a gateway to leave the local network (indirect host). The gateway receiving the data to transmit to another network must then determine the routing from the data’s IP address and an internal table that provides routing information.

If an address is set to all 1s, it applies to all addresses on the network, so an IP address of 32 1s is considered a broadcast message to all networks and all devices. It is possible to broadcast to all machines in a network by altering the local or host address to all 1s so that the address 147.10.255.255 for a Class B network is received by all devices. Coding the address as all 0s refers only to the originating device. The all-zero format is used when the network IP address is not known, but other devices on the network can still interpret the local address. By convention, no local device is given a physical address of 0. It is possible for a device to have more than one IP address if it is connected to more than one network, as is the case with gateways. This is sometimes referred to as being multihomed.

The address 127.0.0.1 is reserved as the loopback address of a device. It is used for test purposes and cannot be assigned as a host ID, but here is a way to configure additional loopback addresses on a router for network-management purposes. Consider a router that has eight interfaces, all of which have a unique IP address. Remote network management systems (NMS) need a target address to reach the router in order to query its MIB. The address used is fundamentally just an open door for the NMS to collect MIB tables regarding the entire router, not just the interface associated with the address. If the circuit is down for the interface that happens to have the target address, data collection is interrupted. Most router vendors offer the capability to configure a virtual interface, using any valid IP address, as a loopback interface for network-management purposes. The main benefit is that it is available as long as the router is operational.

IPv6

IP version 6 (IPv6) was designed to address the issues inherent to IPv4. The major improvement with IPv6 is the capability to handle much larger address spaces, which eliminates any threat of running out of IP addresses. In addition to scalability, IPv6 offers improved security, ease of configuration, and network management. It has been tested on a worldwide, isolated network called 6BONE, which included participants in more than 30 countries.

The major changes brought about by IPv6 are as follows:

• Greater address space. The address space in IPv6 is 128 bits long, compared to IPv4’s 32 bits.

• Stateless addressing. IPv6 networks can automatically route messages using the ICMPv6 discovery messages that send a broadcast to other routers with details of its network.

• Link local address. Automatically configured in the host; valid only in the local physical link.

• Large packet support. Enables packets up to 4GB instead of IPv4’s limit of 64KB.

• Streamlined header that moves nonessential and optional fields to extension headers for increased efficiency in processing at intermediate nodes.

IPv6 addresses are usually written as eight groups of four hexadecimal digits separated by colons. So, if an IPv4 address is 205.154.89.200, an IPv6 address looks like 192a:0d8e:743b:92f2:a083:cf3e:6fe4:8237.

According to specifications in RFC 4292, “IPv6 Addressing Architecture,” long strings of 0s can be compressed using the special syntax ::, as long as it appears only once in an address. The double-colon syntax can also be used for leading or trailing 0s.

Figure 1-8 shows what the IPv6 header looks like.

Figure 1-8

IPv6 header

The header itself is 320 bits long (40 octets) and contains the following:

• Version. 4-bit IP version

• Traffic class. A packet priority value

• Flow label. Used for quality of service (QoS) management (currently unused)

• Payload length. Number of bytes in the payload

• Next header. Next encapsulated protocol (compatible with IPv4 values)

• Hop length. TTL value from IPv4

• Source address. 128-bit IPv6 address

• Destination address. 128-bit IPv6 address