CSCI 4550/8556 Computer Networks Comer, Chapter 18: IP: Internet Protocol Addresses

Motivation One key aspect of a virtual network is a single, uniform address format. We can't use hardware addresses because different technologies have different address formats. The address format must be independent of any particular hardware address format. A sending host puts the destination internet address in a packet. The destination address can be interpreted by any intermediate router. Routers examine the address and forward the packet on to the destination.

TCP/IP Addresses Addressing in TCP/IP is specified by the Internet Protocol (IP). Each host is assigned a 32-bit number. This number is called the IP address or Internet address. It is unique across the entire Internet.

The IP Address Hierarchy Each IP address is divided into a prefix and a suffix. The prefix identifies the network to which computer is attached. The suffix identifies the computer within that network. The IP address format makes routing efficient.

Network and Host Numbers Every network in a TCP/IP internet is assigned a unique network number Each host on a specific network is assigned a host number or host address that is unique within that network. A host's IP address is the combination of the network number (prefix) and the host number (suffix).

Properties of IP Addresses Network numbers are unique. Host addresses may be reused on different networks; combination of network number prefix and host address suffix will be unique. Assignment of network numbers must be coordinated globally. The assignment of host addresses can be managed locally.

Designing the Format of IP Addresses IP designers chose 32-bit addresses. Some bits are allocated to the prefix, and some to the suffix. With a large prefix and small suffix, we have many networks, and few hosts per network. With a small prefix and a large suffix, we have few networks, and many hosts per network. Because of the variety of technologies, the address format needs to allow for both large and small networks.

Classes of IP Addresses The designers chose a compromise - multiple address formats that allow both large and small prefixes. Each format is called an address class. The class of an address is identified by its first few bits.

Using IP Address Classes Classes A, B and C are the primary classes. They are used for ordinary host addressing. Class D is used for multicast, a limited form of broadcast. An Internet hosts join a multicast group. Packets are delivered to all members of group. Routers manage delivery of single packet from its source to all members of a multicast group. Multicast is used for mbone (multicast backbone). Class E is reserved.

Dotted Decimal Notation Addresses in classes A, B and C all break between prefix and suffix on a byte boundary. Dotted decimal notation is a convention for representing 32-bit internet addresses in decimal. To obtain an address in this form, convert each byte of the 32-bit address into decimal, and display the numbers separated by periods (“dots”).

UNOmaha’s IP Addresses UNOmaha has a single class B network: All hosts at UNOmaha have the prefix: cs.unomaha.edu stanw.unomaha.edu apollo.unomaha.edu The suffix bytes are used to determine the local network and host through a technique called subnetting.

Address Classes at a Glance While dotted decimal makes separating a network address from the host address easier, determining the address class is not as easy. Look at the first dotted decimal number, and use this table:

Networks and Hosts in each Class The IP network class scheme does not yield and equal number of networks in each class. In class A, for example, the first bit must be zero, and the next seven bits identify the network (2 7 = 128 possible choices).

Internet Address Allocation Addresses in the Internet are not used efficiently. UNOmaha is typical, using 3, ,000 addresses at a time out of the possible (2 16 ). Large organizations may not be able to get as many addresses in the Internet as they need. Example - UPS needs addresses for millions of computers. Solution - set up a private internet and allocate addresses from the entire 32-bit address space.

Example Select an address class for each network depending on the expected number of hosts. Assign network numbers from the appropriate classes. Then assign host suffixes to form internet addresses for all hosts.

Special IP Addresses

The Berkeley Broadcast Address The first BSD implementation (Berkeley Software Distribution) of UNIX used all zeros for a broadcast address (instead of all ones) This non-standard implementation spread with the popularity of BSD UNIX. This address is still in common use today. “There are two major developments that have come out of Berkeley: BSD UNIX and LSD. This is not a coincidence.” - Anon.

Routers and IP Addressing A router has multiple IP addresses - one for each interface. An IP address depends on the network address. What about routers, connected to two networks? An IP address specifies an interface, or network attachment point, not a computer

Multi-homed Hosts Hosts (that do not forward packets) can also be connected to multiple networks. This can increase their reliability and performance. Multi-homed hosts also have one address for each interface.

Summary A virtual network needs a uniform addressing scheme, independent of hardware. An IP address is a 32-bit address; each interface gets a unique IP address. An IP address is composed of a network address and a host address. Network addresses are divided into three primary classes: A, B and C. Dotted decimal notation is a standard format for Internet addresses: Routers have multiple addresses - one for each interface.