Computer network--An introduction to the Internet layer

Internet Protocol IP is one of the two most important protocols in TCP/IP system. is also one of the most important Internet standard protocols.

• Address Resolution Protocol ARP
• Inter-Network Control Message Protocol ICMP
• Internet Group Management Protocol IGMP
  

Virtual Interconnect Network

There is no single network that adapts to the needs of all users.
In general terms, connecting the networks to each other uses some intermediary devices. There are four intermediate devices depending on the level at which the intermediate device is located:
(1) forwarders used in the physical layer
(2) Bridges or bridges used by the Data link layer
(3) routers used by the network layer
(4) gateways used above the network layer
When the intermediary device is a transponder or bridge, only the network expansion, and from the network layer point of view, is still a network, not called the network interconnection.
The heterogeneous nature of the interconnected physical networks is inherently existent, but we can use IP protocol to make these different performance networks look like a unified network on the network layer. Virtual interconnection networks that use IP protocols can be referred to as IP networks. The advantage of using IP networks is that when the hosts on the IP network communicate, as if they were communicating on a single net, they do not see the specific heterogeneous details of the interconnected networks.

IP address for classification (IPV4)

IP addresses are the most basic concept in the TCP/IP system.

IP address and how it is represented

The whole internet is a single, abstract network. An IP address is a 32-bit identifier that is assigned to each interface on the Internet for every host (or router) that is globally unique.
The method of addressing IP addresses has undergone three historical stages:
(1) IP address of Classification (1981)
(2) Division of Subnets (1985)
(3) Formation of the Network (1993)

IP address of the classification

The IP address is divided into fixed classes, each of which consists of two fixed-length fields, the first of which is the network number that identifies the network on which the host is located, and must be unique across the Internet. The second is the host number , which identifies the host (or router) and must be unique within the network. This shows that the IP address is unique throughout the Internet .
IP Address::={< Network number >,< host number;}

Where a, B, C three is a unicast address, Class D is used for multicast, Class E is later used (1981 later, this figure is incorrect, the E-class address only stipulates the first four bits is 1).
From the IP address structure, the IP address does not only indicate a host, but also indicates which network the host is connected to.

The network number field for Class A addresses is 1 bytes, and only 7 bits are available, and the number of networks you can assign is 126. Because all 0 of the IP address represents "this". Network number 0 is indicated as "This network", network number bit 127 is reserved as the local software loopback test This host process between the use of communication. that's why for 126 reasons.
The network number of Class C addresses is 24 bits (3 bytes), so the host number occupies only 1 bytes, and 253 hosts can be assigned. Because of the IP address, the full 0 host Number field indicates that the IP address is the single network address that the "host" is connected to, and all 1 represents all , so the full 1 host number represents all hosts on that network.

The IP address has the following characteristics:
(1) Each IP address is comprised of a network number and a host number. An IP address is a hierarchical address structure. (for IP address management agencies to allocate IP address, save the router to check the table time)
(2) The IP address is actually an interface that flags a host (or router) and a link. (Note: The router has a minimum of two IP addresses)
(3) According to the Internet, a network is a collection of hosts with the same network number, so several LANs connected with a repeater or bridge are still the same network.
(4) In the IP address, all network numbers assigned to the network are equal.

IP address and hardware address

From a hierarchical point of view, the physical address is the address used by the data link layer and the physical layer, and the IP address is the address used by the network layer and the above layers, which is a logical address .

1. Only IP datagrams can be seen on the internet, which is abstracted from the IP layer.
2. Routers are routed only based on the network number of the IP address of the destination station.
3. On the link layer of the LAN, only Mac frames are visible.
4. The internet, which is abstracted by the IP layer, shields these very complex details from the lower layers. As long as the problem is discussed at the network level, it is possible to study the communication between the host and the host or router using a unified, abstract IP address.

Computer network--An introduction to the Internet layer