* There are two popular approaches to packet switching: the datagram approach and the virtual circuit approach.
* In the datagram approach, each packet is treated independently of all other packets.
* At the network layer, a global addressing system that uniquely identifies every host and router is necessary for delivery of a packet from network to network.
* The Internet address (or IP address) is 32 bits (for IPv4) that uniquely and universally defines a host or router on the internet.
* The portion of the IP address that identifies the network is called the netid.
* The portion of the IP address that identifies the host or router on the network is called the hostid.
* There are five classes of IP 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 a network is easily determined by examination of the first byte.
* Unicast communication is one source sending a packet to one destination.
* Multicast communication is one source sending a packet to multiple destinations.
* Subetting divides one large network into several smaller ones.
* 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, 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.
* 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.
