CompTIA Network+ Rapid Review: Network Concepts

  • 12/15/2012

Objective 1.3: Explain the purpose and properties of IP addressing

IP addressing is one of the fundamental functions of the TCP/IP protocol suite and the network layer IP. Every device on an internetwork must have a unique IP address, so that IP can address packets specifically to it. IP addresses specify both the network on which the device is located and the device itself, called a host, on that particular network. Routers use the network identifier to forward packets to the correct network, and the router on the destination network uses the host identifier to forward the packets to the correct device.

Exam need to know

  • Explain the intended purpose and properties of now-obsolete IP address classes.

    For example: Which IP address class provided the largest number of hosts per subnet?

  • Explain the purpose and properties of Classless Inter-Domain Routing (CIDR).

    For example: How many bits are allocated for the host identifier in the network address?

  • Explain the purpose and properties of IPv4 and IPv6 formatting.

    For example: What is the largest possible value for each of the four decimal numbers in an IPv4 address?

  • Explain the purpose and properties of the MAC address format.

    For example: What is the term used for the first three bytes of a MAC address?

  • Explain the purpose and properties of subnetting.

    For example: How many hosts can you create on a subnet with the mask

  • Explain the purpose and properties of multicasts, unicasts, and broadcasts.

    For example: What is the standard MAC address value used for a broadcast transmission?

  • Explain the purpose and properties of APIPA.

    For example: What is the IPv4 network used by default for Automatic Private IP Addressing?

IP address classes

IPv4 addresses contain both a network identifier and a host identifier, which means that some of the 32 bits in the address specify the network on which the host is located and the rest of the bits identify the specific host on that network. However, the division between the network identifier bits and the host identifier bits is not always in the same place. The original IP standard defined three primary classes of IP addresses: A, B, and C, which provided support for networks of different sizes, as shown in Figure 1-3.

Figure 1-3

Figure 1-3 The three primary classes of IPv4 addresses.

The characteristics of these three address classes are listed in Table 1-1.

Table 1-1 IPv4 address classes.





First bit values (binary)




First byte value (decimal)

0 -127



Number of network identifier bits




Number of host identifier bits




Number of possible networks




Number of possible hosts




The “First bit values” row in the table specifies the values that the first one, two, or three bits of an address in each class must have. Early TCP/IP implementations used these bit values to determine the class of an address.

For web servers and other computers to be accessible by clients on the Internet, they must have public IP addresses, that is, addresses registered with an authority, such as an Internet service provider (ISP). For workstations and other computers that do not require an Internet presence, administrators typically use private IP addresses, which are freely available for use on any network and are not registered as belonging to any particular organization.

The private address ranges for each class are as follows:

  • Class A through
  • Class B through
  • Class C through

True or false: You cannot assign all of the possible values in a given address class to network devices.

Answer: True. The host identifier values in each address class consisting of all zeroes and all ones are reserved; you cannot assign them to hosts. The all zeroes address identifies the network itself and the all ones address is the broadcast address for the network.

True or false: A web server must have a public IP address to be accessible by clients on the Internet.

Answer: True. Public, or registered, IP addresses are assigned to particular organization and reserved for use by one host on the Internet.

Classless inter-domain routing (CIDR)

There are many networks that have more than the 254 hosts provided by a Class C address, and there are none that have the 16 million provided by a Class A. The classful IP addressing system, therefore, proved to be wasteful as the IP address space grew crowded. CIDR is a subnetting method that enables administrators to place the division between the network bits and the host bits anywhere in the address, not just between octets. This makes it possible to create networks of almost any size.

CIDR also introduced a new notation for network addresses. A standard IPv4 network address is followed by a forward slash and a numeral specifying the size of the network identifier. For example, represents an address that uses a 24-bit network identifier, leaving the other 8 bits for up to 254 host identifiers, which would formerly be known as a Class C address.

True or false: Classless IP addresses use the first few binary bits of the network identifier to specify the size of the network.

Answer: False. In a classless address, the size of the network is indicated by the suffix, or by the use of a subnet mask.

True or false: In the classless address, the number 24 specifies how many hosts you can create on the network.

Answer: False. The number 24 indicates the number of bits in the network identifier. There are therefore 8 host bits, allowing a maximum of 254 hosts on the network.

IPv4 and IPv6 address formatting

The original IP protocol standard calls for 32-bit IP addresses, but the depletion of the IPv4 address space led to the development of IPv6, which uses 128-bit addresses. The IP addresses used in networks around the world are currently in the midst of a lengthy conversion from IPv4 to IPv6.

An IPv4 address is a 32-bit value that contains both a network identifier and a host identifier. The address is notated by using four decimal numbers ranging from 0 to 255, separated by periods, as in This is known as dotted decimal notation.

IPv6 addresses use a notation called colon-hexadecimal format, which consists of eight 16-bit hexadecimal numbers, separated by colons, as in the following example:


When an IPv6 address has two or more consecutive 8-bit blocks of 0s, you can replace them with a double colon. You can also remove the leading 0s in any block where they appear, as follows:


True or false: The hexadecimal value 21cd:53::e8bb::3c:c394 is a valid IPv6 address.

Answer: False. A valid IPv6 address can only have one double colon in it.

MAC address formatting

The first three bytes of a MAC address, called the organizationally unique identifier (OUI), consist of a value assigned to the hardware manufacturer by the Institute of Electrical and Electronics Engineers (IEEE). The second three bytes consist of a unique value assigned by the manufacturer to each individual device.

True or false: Two computers can have the same OUI.

Answer: True. The OUI is a value assigned to a manufacturer of network interface adapters, and all of the adapters produced by that manufacturer will have MAC addresses with identical OUIs. Only the second three bytes of the MAC address on every adapter must be unique.

True or false: The Ipconfig.exe program on a Windows computer displays the MAC address assigned to the network interface adapter.

Answer: True. In addition to TCP/IP configuration settings, Ipconfig.exe identifies each of the network interface adapters in the computer and displays their MAC addresses, as in the third line of the following display.

Connection-specific DNS Suffix  . : zacker.local
Description . . . . . . . . . . . : Realtek PCIe GBE Family Controller
Physical Address. . . . . . . . . : 60-EB-69-93-5E-E5
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::7441:4473:f204:ec1d%10(Preferred)
IPv4 Address. . . . . . . . . . . :
Subnet Mask . . . . . . . . . . . :
Lease Obtained. . . . . . . . . . : Sunday, April 15, 2012 1:11:50 PM
Lease Expires . . . . . . . . . . : Friday, April 27, 2012 1:11:48 PM
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . :
DHCPv6 IAID . . . . . . . . . . . : 241232745
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-14-81-CC-39-60-EB-69-93-5E-E5
DNS Servers . . . . . . . . . . . :
Primary WINS Server . . . . . . . :
NetBIOS over Tcpip. . . . . . . . : Enabled

IP address subnetting

When the wastefulness of classful IP addressing was recognized, the designers of the IP protocol developed a system for subdividing network addresses by creating subnets within them. A subnet is simply a subdivision of a network address that administrators can use to represent a part of a larger network, such as one LAN on an internetwork or the client of an ISP. Thus, a large ISP might have a Class A address registered to it, and it might allocate sections of that network address to its clients in the form of subnets.

To understand the process of creating subnets, you must understand the function of the subnet mask. TCP/IP systems at one time recognized the class of an address simply by examining the values of its first three bits. Today, when you configure the TCP/IP client on a computer, you assign it an IPv4 address and a subnet mask. The subnet mask is a 32-bit value that specifies which bits of the IP address are the network identifier and which bits are the host identifier. For example, the subnet mask, in binary form, is 24 ones and eight zeroes. The ones are the network identifier bits and the zeroes are the host identifier bits.

True or false: To create eight-bit subnets on a Class A network address, you would use a subnet mask value of

Answer: True. The subnet mask for a Class A address is Borrowing eight bits from the host identifier to create subnets gives you a 16-bit network identifier. The subnet mask, therefore, consists of 16 ones and 16 zeroes, in binary form, or in decimal form.

True or false: The boundary between the network identifier and the host identifier in a subnetted IPv4 address must fall between bytes.

Answer: False. Subnets can be any size, so the boundary between the network and host identifiers can theoretically fall between any two bits.

Multicasts, unicasts, and broadcasts

IPv4 supports three basic types of addresses, as follows:

  • Unicast A one-to-one transmission sent to an IP address with a specific host identifier, anywhere on the internetwork.
  • Broadcast A one-to-many transmission sent to an IP address with a host identifier that consists of all 1s. Broadcast transmissions are received and processed by all of the hosts on the local network.
  • Multicast A one-to-many transmission sent to a specially-allocated multicast IP address. Multicast addresses are targeted at specific groups of hosts, which can be scattered around the internetwork.

True or false: Registration of hosts in multicast groups is handled by the Internet Control Message Protocol (ICMP).

Answer: False. The protocol that hosts use to register themselves in multicast groups is called the Internet Group Management Protocol (IGMP).

True or false: Both MAC addresses and IPv4 addresses support broadcast transmissions, but IPv6 addresses do not.

Answer: True. MAC addresses and IPv4 addresses consisting of all ones (ffffffffffff or, respectively) cause a transmission to be sent to all of the local network devices. IPv6, however, has no broadcast addresses; it uses multicasts and a new type of transmission called an anycast, instead.

Automatic private IP addressing

Automatic Private IP Addressing (APIPA) is a DHCP failover mechanism used by all of the current Windows operating systems. When a device fails to locate a DHCP server on the network, APIPA takes over and automatically assigns an address on the network to the computer. The system then uses the Address Resolution Protocol (ARP) to ensure that no other computer on the local network is using the same address.

For a small network that consists of only a single, unrouted LAN, APIPA is a simple and effective alternative to installing a DHCP server, as it creates and assigns addresses that are all on the same subnet.

True or false: Two computers on the same network that assign themselves addresses using APIPA cannot communicate with each other.

Answer: False. APIPA assigns addresses that are all on the same IP subnet, and the systems use ARP to confirm that their addresses are unique. Therefore, two systems on the same network with APIPA addresses can communicate with each other.

True or false: APIPA is capable of assigning both IPv4 and IPv6 addresses.

Answer: False. APIPA can only assign IPv4 addresses. IPv6 has its own mechanism for self-assigning addresses, called stateless address autoconfiguration.

Can you answer these questions?

Find the answers to these questions at the end of this chapter.

  1. Which IP address class provides the largest number of hosts per subnet?
  2. What subnet mask must you use for a network with the address
  3. What subnet does APIPA use when assigning IP addresses?
  4. The link local unicast addresses generated by the IPv6 stateless address autoconfiguration process use the network address fe80:0000:0000:0000/64. What is the most compact allowable form of this address?
  5. What is the OUI in the following MAC address: 60-EB-69-86-3A-C7?

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