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Network Layer: Logical Addressing

Definition of Terms

• At the network layer, a global identification system that uniquely identifies every host and router is necessary for delivery of a packet from host to host (network to network).
• An IPv4 address is 32 bits long and uniquely and universally defines a host or router on the Internet. 
• The IPv4 addresses are unique and universal.
• The address space of  IPv4 is 2³² or 4,294,967,296.
• An address space is the total number of addresses used by the protocol. If a protocol uses N bits to define an address, the address space is 2N because each bit can have two different values (0 or 1) and N bits can have 2N values.
• In binary notation, the IPv4 address is displayed as 32 bits. Each octet is often referred to as a byte. So it is common to hear an IPv4 address referred to as a 32-bit address or a 4-byte address.
• Dotted-Decimal Notation - to make the IPv4 address more compact and easier to read, Internet addresses are usually written in decimal form with a decimal point (dot) separating the bytes.
• In classful addressing, the portion of the IP address that identifies the network is called the netid.
• In classful addressing, the portion of the IP address that identifies the host or router on the network is called the hostid.
• An IP address defines a device's connection to a network.
• There are five classes in IPv4 addresses. Classes A, B, and C differ in the number of hosts allowed per network. Class D is for multicasting and Class E is reserved.
• The class of an address is easily determined by examination of the first byte.
• Addresses in classes A, B, or C are mostly used for unicast communication.
• Addresses in class D are used for multicast communication.
• Unicast communication is one source sending a packet to one destination.
• Multicast communication is one source sending a packet to multiple destinations.
• Subnetting divides one large network into several smaller ones, adding an intermediate level of hierarchy in IP addressing.
• Subnetting adds an intermediate level of hierarchy in IP addressing. 
• Default masking is a process that extracts the network address from an IP address. 
• Subnet masking is a process that extracts the subnetwork address from an IP address 
• Supernetting combines several networks into one large one.
• In classless addressing, we can divide the address space into variable-length blocks. 
• In classless addressing, there are variable-length blocks that belong to no class. The entire address space is divided into blocks based on organization needs. 
• The first address and the mask in classless addressing can define the whole block. 
• A mask can be expressed in slash notation which is a slash followed by the number of 1s in the mask.
• The mask in classless addressing is expressed as the prefix length (ln) in CIDR notation.
• To find the first address in a block, we set the rightmost 32 - n bits to O.
• To find the number of addresses in the block, we calculate 2^{32 - n}, where n is the prefix length.
• To find the last address in the block, we set the rightmost 32 - n bits to 1s.
• In IPv4 addressing, a block of addresses can be defined as x.y.z.t /n in which x.y.z.t defines one of the addresses and the /n defines the mask.
• The number of addresses in the block can be found by using the formula 2³²⁻ⁿ.
• Subnetting increases the value of n.
• Every computer attached to the Internet must know its IP address, the IP address of a router, the IP address of a name server, and its subnet mask (if it is part of a subnet). 
• DHCP is a dynamic configuration protocol with two databases. 
• The DHCP server issues a lease for an IP address to a client for a specific period of time.
• Network address translation (NAT) allows a private network to use a set of private addresses for internal communication and a set of global Internet addresses for external communication. 
• NAT uses translation tables to route messages.
• The IP protocol is a connectionless protocol. Every packet is independent and has no relationship to any other packet. 
• Every host or router has a routing table to route IP packets.
• In next-hop routing, instead of a complete list of the stops the packet must make, only the address of the next hop is listed in the routing table.
• In network-specific routing, all hosts on a network share one entry in the routing table.  
• In host-specific routing, the full IP address of a host is given in the routing table. 
• In default routing, a router is assigned to receive all packets with no match in the routing table.
• A static routing table's entries are updated manually by an administrator. 
• Classless addressing requires hierarchial and geographic routing to prevent immense routing tables.
• The global authority for address allocation is ICANN.  ICANN normally grants large blocks of addresses to ISPs, which in turn grant small subblocks to individual customers.
• IPv6 addresses use hexadecimal colon notation with abbreviation methods available.
• There are three types of addresses in IPv6: unicast, anycast, and multicast. 
• In an IPv6 address, the variable type prefix field defines the address type or purpose.
• An IPv4 address is 32 bits long. An IPv6 address is 128 bits long.
• Classful addressing assigns an organization a Class A, Class B, or Class C block of addresses. Classless addressing assigns an organization a block of contiguous addresses based on its needs.
• A block in class A address is too large for almost any organization. This means most of the addresses in class A are wasted and not used. A block in class C is probably too small for many organizations.
• The network address in a block of addresses is the first address. The mask can be ANDed with any address in the block to find the network address.
• Multicast addresses in IPv4 are those that start with the 1110 pattern. Multicast addresses in IPv6 are those that start with the 11111111 pattern.

There are three restrictions in classless addressing:

• a. The number of addresses needs to be a power of 2. 
• b. The mask needs to be included in the address to define the block.
• c. The starting address must be divisible by the number of addresses in the block.

Finding the classes in binary and dotted-decimal notation

Number of blocks and block size in classful IPv4 addressing

Default masks for classful addressing

IPv6 address in binary and hexadecimal colon notation

Note: You can proceed to take the multiple choice exam regarding this topic. Virtual-Circuit Networks: Frame Relay and ATM - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com