IPv4 Addressing CCIE Bootcamp Training in Delhi Gurgaon

Network Bulls
www.networkbulls.com
Best Institute for CCNA CCNP CCSP CCIP CCIE Training in India
M-44, Old Dlf, Sector-14 Gurgaon, Haryana, India
Call: +91-9654672192

This section covers the IPv4 address classes, private addressing, and NAT. The IPv4 address space was initially divided into five classes. Each IP address class is identified by the initial bits of the address. Classes A, B, and C are unicast IP addresses, meaning that the destination is a single host. IP Class D addresses are multicast addresses, which are sent to multiple hosts. IP Class E addresses are reserved. Private addresses are selected address ranges that are reserved for use by companies in their private networks. These private addresses are not routed in the Internet. NAT translates between private and public addresses.

An IP addresses is a unique logical number to a network device or interface. An IP address is 32 bits in length. To make the number easier to read, the dotted-decimal format is used. The bits are combined into four 8-bit groups, each converted into decimal numbers—for example, 10.1.1.1. If you are not familiar with binary numbers, Appendix B contains a review of binary and hexadecimal number manipulation.

The following example shows an IP address in binary and decimal formats:

Binary IP address: 01101110 00110010 11110010 00001010
Convert each byte into decimal.

For the first octet:
01101110
0+64+32+0+8+4+2+0 = 110
01101110 = 110

For the second octet:
00110010
0+0+32+16+0+0+2+0 = 50
00110010 = 50

For the third octet:
11110010
128+64+32+16+0+0+2+0 = 242
11110010 = 242

For the fourth octet:
00001010
0+0+0+0+8+0+2+0 = 10
00001010 = 10

The IP address is 110.50.242.10.

IPv4 Address Classes

IPv4 addresses have five classes—A, B, C, D, and E. In classful addressing, the most significant bits of the first byte determine the address class of the IP address. Table 7-8 shows the high-order bits of each IP address class.

Table 7-8. High-Order Bits of IPv4 Address Classes

Address Class	High-Order Bits[*]
A	0xxxxxxx
B	10xxxxxx
C	110xxxxx
D	1110xxxx
E	1111xxxx

^[*] x can be either 1 or 0, regardless of the address class.

Again, the IPv4 Class A, B, and C addresses are unicast addresses. Unicast addresses represent a single destination. Class D is for multicast addresses. Packets sent to a multicast address are sent to a group of hosts. Class E addresses are reserved for experimental use. IANA allocates the IPv4 address space. IANA delegates regional assignments to Regional Internet Registries (RIR). The five RIRs are

• ARIN (American Registry for Internet Numbers)
• RIPE NCC (Reseaux IP Europeens Network Control Center)
• APNIC (Asia Pacific Network Information Center)
• LACNIC (Latin America and Caribbean Network Information Center)
• AfriNIC (African Network Information Centre)

Updates to the IPv4 address space can be found at http://www.iana.org/assignments/ipv4-address-space.

The following sections discuss each of these classes in detail.

Class A Addresses

Class A addresses range from 0 (00000000) to 127 (01111111) in the first byte. Network numbers available for assignment to organizations are from 1.0.0.0 to 126.0.0.0. Networks 0 and 127 are reserved. For example, 127.0.0.1 is reserved for localhost or host loopback. A packet sent to a localhost address is sent to the local machine.

By default, for Class A addresses, the first byte is the network number, and the three remaining bytes are the host number. The format is N.H.H.H, where N is the network part and H is the host part. With 24 bits available, there are 2²⁴ – 2 = 16,777,214 IP addresses for host assignment per Class A network. We subtract two for the network number (all 0s) and broadcast address (all 1s). A network with this many hosts will surely not work with so many hosts attempting to broadcast on the network. This section discusses subnetting later as a method of defining smaller networks within a larger network address.

Class B Addresses

Class B addresses range from 128 (10000000) to 191 (10111111) in the first byte. Network numbers assigned to companies or other organizations are from 128.0.0.0 to 191.255.0.0. This section discusses the 16 networks reserved for private use later.

By default, for Class B addresses, the first two bytes are the network number, and the remaining two bytes are the host number. The format is N.N.H.H. With 16 bits available, there are 2¹⁶ – 2 = 65,534 IP addresses for host assignment per Class B network. As with Class A addresses, having a segment with more than 65,000 hosts broadcasting will surely not work; you resolve this issue with subnetting.

Class C Addresses

Class C addresses range from 192 (11000000) to 223 (11011111) in the first byte. Network numbers assigned to companies are from 192.0.0.0 to 223.255.255.0. The format is N.N.N.H. With 8 bits available, there are 2⁸ – 2 = 254 IP addresses for host assignment per Class C network. H = 0 is the network number; H = 255 is the broadcast address.

Class D Addresses

Class D addresses range from 224 (11100000) to 239 (11101111) in the first byte. Network numbers assigned to multicast groups range from 224.0.0.1 to 239.255.255.255. These addresses do not have a host or network part. Some multicast addresses are already assigned; for example, 224.0.0.10 is used by routers running EIGRP. A full list of assigned multicast addresses can be found at http://www.iana.org/assignments/multicast-addresses.

Class E Addresses

Class E addresses range from 240 (11110000) to 254 (11111110) in the first byte. These addresses are reserved for experimental networks. Network 255 is reserved for the broadcast address, such as 255.255.255.255. Table 7-9 summarizes the IPv4 address classes. Again, each address class can be uniquely identified in binary by the high-order bits.

Table 7-9. IPv4 Address Classes

Address Class	High-Order Bits	Network Numbers
A	0xxxxxxx	1.0.0.0 to 126.0.0.0[*]
B	10xxxxxx	128.0.0.0 to 191.255.0.0
C	110xxxxx	192.0.0.0 to 223.255.255.0
D	1110xxxx	224.0.0.1 to 239.255.255.255
E	1111xxxx	240.0.0.0 to 254.255.255.255

^[*] Networks 0.0.0.0 and 127.0.0.0 are reserved as special-use addresses.

IPv4 Private Addresses

Some network numbers within the IPv4 address space are reserved for private use. These numbers are not routed on the Internet. Many organizations today use private addresses in their internal networks with NAT to access the Internet. (NAT is covered later in this chapter.) Private addresses are explained in RFC 1918, Address Allocation for Private Internets, published in 1996. Private addresses were one of the first steps dealing with the concern that the globally unique IPv4 address space would become exhausted. The availability of private addresses combined with NAT reduces the need for organizations to carefully define subnets to minimize the waste of assigned, public, global IP addresses.

The IP network address space reserved for private internets is 10/8, 172.16/12, and 192.168/16. It includes one Class A network, 16 Class B networks, and 256 Class C networks. Table 7-10 summarizes private address space. Large organizations can use network 10.0.0.0/8 to assign address space throughout the enterprise. Midsize organizations can use one of the Class B private networks 172.16.0.0/16 through 172.31.0.0/16 for IP addresses. The smaller Class C addresses, which begin with 192.168, support only up to 254 hosts each.

Table 7-10. IPv4 Private Address Space

Class Type	Start Address	End Address
Class A	10.0.0.0	10.255.255.255
Class B	172.16.0.0	172.31.255.255
Class C	192.168.0.0	192.168.255.255

NAT

NAT devices convert internal IP address space into globally unique IP addresses. NAT was originally specified by RFC 1631; the current specification is RFC 3022. Companies use NAT to translate internal private addresses to public addresses.

The translation can be from many private addresses to a single public address or from many private addresses to a range of public addresses. When NAT performs many-to-one, the process is called port address translation (PAT) because different port numbers identify translations.

As shown in Figure 7-3, the source addresses for outgoing IP packets are converted to globally unique IP addresses. The conversion can be configured statically, or it can dynamically use a global pool of addresses.

NAT has several forms:

• Static NAT— Maps an unregistered IP address to a registered IP address; it is configured manually.
• Dynamic NAT— Dynamically maps an unregistered IP address to a registered IP address from a pool (group) of registered addresses. The two subsets of dynamic NAT are overloading and overlapping:

  - Overloading— Maps multiple unregistered IP addresses to a single registered IP address by using different ports. This is also known as PAT, single-address NAT, or port-level multiplexed NAT.

  - Overlapping— Maps registered internal IP addresses to outside registered IP addresses. It can also map external addresses to internal registered addresses.

When designing for NAT, you should understand the following terminology:

• Stub domain— The internal network that might be using private IP addresses.
• Public network— Outside the stub domain, it resides in the Internet. Addresses in the public network can be reached from the Internet.
• Inside local address— The real IP address of the device that resides in the internal network. This address is used in the stub domain.
• Inside global address— The translated IP address of the device that resides in the internal network. This address is used in the public network.
• Outside global address— The real IP address of a device that resides in the Internet, outside the stub domain.
• Outside local address— The translated IP address of the device that resides in the Internet. This address is used inside the stub domain.

Figure 7-4 illustrates the terms described in the list. The real IP address of the host in the stub network is 192.168.10.100; it is the inside local address. The NAT router translates the inside local address into the inside global address (200.100.10.100). Hosts located in the Internet have their real IP address (outside global address) translated; in the example, 30.100.2.50 is translated into the outside local address of 192.168.100.50.