Computer network Overview

Two ways to define a computer network:

1. Generally, the dispersed multiple computers, terminals and external equipment with communication lines to communicate with each other, and the computer hardware, software and data resources can be used together, the entire system to achieve resource sharing is called computer network.

2. Computer network (Computer Network) : Refers to the use of certain communication lines, geographically dispersed, with independent autonomy of the computer system, and so on, under the constraints and control of a certain communication protocol, the realization of data exchange communication system.

TCP/IP5 Layer Model:

1. Application Layer (Application layer)

Direct service to the user's application process

2. Transport Layer (Transport Layer)

Responsible for providing services for communication between processes in two hosts

3. Network Layer (Network layer)

Responsible for providing communication services for different hosts on the packet switching network

4. Data Link Layer (Data link layer)

Reliably transfer data from the network layer to the target network layer of neighboring nodes

Common data Link layer devices have network cards, bridges, and two-layer switches.

Installed on the computer, used to connect the computer network, is the most basic network equipment in the computer network.

5. Category: Wired Ethernet card, WLAN Wireless Card

Physical Layer (Physical layer)

Delivering data paths and transmitting data to data-end devices

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OSI7 Layer Model:

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Data formats that are transmitted on each layer

the units of data transferred between peer layers are called PDU(Protocol data Unit)

Physical layer: Transmitted in the most primitive bitstream format, PDU is bit

Data Link layer: The unit of transmission is a frame, one frame consists of multiple bits, but the size of a frame must be an integer byte. The true size of the different protocols is not the same.

Network layer: The unit of transmission is a grouping (or a package), a grouping can contain multiple true, and the size of the grouping depends on the different protocols.

Transport layer: in In TCP/IP protocol system,TCP is transmitted in data segment, andUDP is transmitted in datagram Unit .

Session layer, Presentation layer, Application layer: transmission in a specific data message unit

Communication subnets and resource subnets:

1. Communication subnets:

A set of devices and software that implements network communication functions in computer networks is called network communication subnet.

The set of devices and software that implements the resource sharing function in the network is called the resource subnet.

Network Communication functions of the equipment and the collection of software , communication equipment, network communication protocol, communication control software, is the inner layer of the network, responsible for the transmission of information. Mainly for the user to provide data transmission, transfer, processing, transformation, etc.

Repeater device

Hubs

Switch

Communication cable

Router

Various gateway hardware devices, etc.

2. Resource subnets:

resource subnets are mainly composed of network servers, workstations, shared printers and other devices and related software. The principal of a resource subnet is a network resource device, including:

User computers (also known as workstations)

Networked Storage Systems

Network printers

Network data devices that run independently

Network Terminal

Server

Various software resources running on the network

Data resources, etc.

Network Bridge working mechanism:

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Two-layer switch working mechanism:

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VLAN Basic Concepts:

The main purpose of a VLAN is to divide a large switching network into smaller switching networks.

Narrow the broadcast domain to reduce the impact of broadcast traffic on the entire switching network in a two-tier network.

VLANs are two-layer protocols, virtual VLANs, or logical attributes that determine that there are no physical two-tier connections between these VLANs (only logical connections), each independent of each other, equivalent to a separate two-layer switched network. Each VLAN member communicates with a member of another VLAN through a three-layer SVI interface that both parties act as their respective VLAN member gateways .

1. Line switching

" line switching "(circuitswitching, also known as "circuit switching") is the most primitive way of exchanging data, and it is a way of exchanging data between two parties in a network using a switchable physical communication line to communicate directly.

Line Exchange "is a connection-oriented service, two computers through the communication subnet for data exchange before the first to establish an actual physical connection in the communication subnet (usually controlled by a switching circuit). The main feature is that before the data exchange before the need to set up a dedicated data path between a pair of users, in the entire data transmission process through the line establishment, data transmission and line release of the three

2. Storage - forwarding

Store - and-forward "is a network node application program will be the path of the data stream according to the transmission unit (can be a packet of messages or packets) received and stored, a data unit after receiving, according to the relevant routing algorithm select a suitable routing path to the data forwarded out, Provides a transport path to the data stream on a logical (not physical) line

① Message Exchange

"Message Exchange " means a data exchange method in which information is stored-forwarded as a unit of " message "(message, a piece of information of complete data) . In the "message exchange" mode, "message" is the network exchange and transmission of the data unit, that is, the site to send a data block, contains the complete data to be sent, the length may be inconsistent, the length is unlimited and variable.

② packet Switching

⑴ Datagram Service

the groupings are called "datagrams" (datagram). It uses routing technology to forward the packet header with the source node and the destination node IP address, and then through the routing technology level to forward the data.

⑵ Virtual Circuit Service

The virtual channel established is called "virtual circuit" ( Virtual circuit,VC). Instead of relying on routing technology, a virtual communication channel is established between the source node and the destination node before the data packet is transferred, and then the data is grouped from the virtual channel to the destination node.

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IPV4 Message Format:

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1. version: 4 bits, such as 0100 for IPv4,0110 said IPv6

2. Header Length: The optional field sizeof the packet changes (20-60), that is, the fixed part + optional part

First Ministerial degree accounted for 4 bits, the maximum value that can be represented is a unit of a number, note: This field represents the units of the numbers are the four digits ( a three-bit word length is 4 bytes ), so the maximum of the first ministerial degree of the IP is a number of bytes.

3. Service Type: priority,TOS, for classification, identification of packet priority

4. Display Congestion notification (ECN): for congested signals

5. Total length -- -----the length of the sum of the header and data, in bytes, so the maximum length of the datagram is 65535 bytes.

The total length must not exceed the maximum transmission unit MTU .

6. identifier: Together with the Tag field and the Segment offset field for packet fragmentation (if the packet size is greater than the data link MTU value, then the packet needs to be decomposed and forwarded)

7. Mark Field: 3 bits in length, first bit useless, second bit is non-segmented DF bit (when df=1 indicates that packets are not allowed to be fragmented, thus discarded, can be used to test the MTU value) with Ping can be set to DF

Third place ( MF ): When the router segments the MF, the router fragments the packet, except that the last MF bit is set to 0 , all the other segmented MF values are 1, The recipient receives a segment with a MF bit of 0

8. Segment offset: The slice offset (in decimal ) indicates the relative position of a piece in the original group after the Shard in a longer grouping. The slice offset is offset in 8 bytes. The function is to prevent the wrong sequence, so that the recipient receives the data segment reorganization in the correct order

9. time to Live:TTL prevents packets from being forwarded on the network indefinitely, to 0 drops

The Protocol : Protocol (8 -bit ) field indicates what protocol is used for the data carried by this datagram, so that the IP layer of the destination host gives the data part to which process. Protocol number 1 stands for ICMP 4 for IP 6 is TCP + UDP is the GRE-in OSPF

First Checksum: Error correction, verifying the header but not verifying the data section

Source address and Destination address: sender, Recipient

available options: for testing

Loose Source route selection (loose): Allows multiple routers to be crossed

Strict Source route selection (strict): The next hop is forwarded in the list, not discarded

Record routing: Records the interface information on the bidirectional path of a packet, which is the interface that passes through

Timestamp : Record the time (which interface of which router was traversed)

padding: Used to add 0 to the top of the optional field, to ensure that the header length is a multiple of three

ARP Overview:

ARP is the protocol that resolves an IP address to an Ethernet MAC address (or physical address).

ARP is an independent three-layer protocol

ARP messages are transmitted to the data link layer without having to pass the IP protocol encapsulation, but instead generate their own messages directly.

ARP Mapping Table

the corresponding relationship between IP address and MAC address:

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ARP Address Resolution principle:

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ICMP(internetcontral message Protocol)Internet Control Message Protocol

a sub-protocol in an IP protocol cluster

used in Transfer control messages between IP hosts and routers

A control message is a message that the network is not accessible, whether the host is available, whether the route is available, etc.

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Network address

The address used to identify a class or no-Class network, the first of the corresponding network or subnet IPv4 Address, the " host ID" section of the IPv4 address that is all 0

Host Address

In addition to the network address and broadcast address of the two end of the address, all the other addresses in the middle are the host address, can be directly assigned to the host to use

Broadcast Address

is the last one in a class or class-free network IPV4 address, the "host ID" section of the IPv4 address that is all 1

Example title:

1, a company originally used 192.168.1.0/24 This standard network, now want to each department in the company (a total of 6 ) a separate subnet, one of the largest departments to allocate IPV4 The number of addresses is not more than one. Subnet masks, address ranges, network addresses, and broadcast addresses for each subnet.

Answer:

Mask:255.255.255.224---2^5>25, so the host bit is 5, the network bit is 3, that is 11100000=224

address range:0-31,32-63,64-95,96-127,128-159,160-191,192-223,224-255

Network address:. 0.32.64.96.128.160.192.224

Broadcast address: 31 63 95 127 159 191 223 255

Transport Layer Protocol resolution please refer to another Blog Transport Layer protocol

This article is from the "After Tomorrow" blog, please be sure to keep this source http://leeyan.blog.51cto.com/8379003/1697412

Computer network Overview