###### tags: `計算機概論`
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# Networking and the internet(I)
## Network Classifications
* Scope
* LAN: Local area network.
* MAN: Metropolitan area network.
* WAN: Wide area network.
* Ownership
* Closed
* Opened
* Topology (拓樸學)
* Bus:
* Ring:
* Star:
## Protocols
* Token Ring
* Popular in ring topology.
* Token and messages are passed in one direction.
* Only the machine which gets the token can transmit its message.
* CSMA/CD (Carrier sense, mutiple access with collision detection)
* Popular in bus topology.
* Broadcasting.
* When collision, both machines wait for a random time before trying again.
* CSMA/CA (Carrier sense, mutiple access with collision avoidence)
* Popular in wireless Ethernet.
* Broadcasting.
* Detect if a channel is idle, if so, wait for a random time and then detect again. If the channel is still idle, then start sending.
## Wireless and Access Point(AP)
* Wi-Fi(wireless fidelity)
* IEEE 802.11-Standard for wireless network(b, g, i, n, ac...)
## Repeater，Bridge，Switch(Compatible Networks)
Reference: https://notfalse.net/66/repeater-hub-bridge-switch
* The protocol between these devices have to be the same.
* Repeater(Amplifier)
* Passing through messages.
* Connecting two compatible networks
* Bridge
* Only passing those messages addressed to the other side.
* Connecting two compatible networks more efficiently.
* Switch(Multiple Bridge)
* A bridge with multiple connections.
* Connecting mutiple compatible networks efficiently.
## Router(Incompatible Networks)
* Router
* Building a network of networks.(The protocol can be different)
* Most come with firewall management.
## Interprocess Communication
* Server-Client
* One server, several clients.
* Clients initiate communications by sending requests.
* P2P(Peer-to-Peer)
* Two processes communicating as equals.
* The most popular distribution mode nowadays.
## Distributed Systems
* Infrastructure can be provided by standardized toolkits.
* Ex: Java Beans，.NET framework.
## Internet Architecture
* Domain
* A network or an internet controlled by one single authority.
* ICANN(Internmet corporation for assigned names and numbers)
* Supervise the registration of doimains
* Gateway
* A router that connects a domain to the rest of the Internet.
## Internet Composition
* Internet Service Provider(ISP)
* Allow customer to connect their domain to the ISP's equiment or join the domain already established by the ISP.
## IP Addresses
* IP(internet protocol)address
* IPv4 -> 32bits(all are allocated already)
* IPv6 -> 128bits
## Name Server vs Domain Server
* Name server:
* Name servers organize and route traffic across the internet to get you to the right server.
* Domain Name Server(DNS)
* A server that provide you with server information.
## Internet Application
* VoIP(Voice over Internet protocol)
* email(electronic mail)
* FTP(file transfer protocol)
* telnet and ssh(secure shell)
* P2P
## Browsers
* HTTP(Hyper text transfer protocol)
* URL(uniform resource locator)
* How a browser get access to a webpage(document):
## Client-side and Server-side
* Client-side(The program is executed on one's pc)
* Java applets
* Javascript
* Flash
* Server-side(The program is executed on the server)
* CGI
* Servlets
* PHP
* Online game use both techniques.
## Internet Protocol
Reference:https://ithelp.ithome.com.tw/articles/10259569?sc=pt
* Layers
* OSI Model
* Reference: https://en.wikipedia.org/wiki/OSI_model


* TCP/IP Model
* Application: constructs message into packets.
* Transport: chops message into packages.
* Network: handles routing through the Internet.
* Link handles actual transmission of packets.


* Port
* Incoming messages are delivered to different applications by unique port numbers.
## TCP/IP
* TCP and UDP(Transport Layer)
* TCP(transmission control protocol):handshake berfore sending packets. => slow but reliable.
* UDP(user datagram protocol): directly sending packets. => fast but not reliable.
* IP(NEtwork Layer)
* routing based on IP(IPv4, IPv6)
* TCP and IP are two protocols. TCP/IP is a collection of protocols more than just TCP and IP.
## Security
* Attacks
* Malware
* Denial of Service(Dos)
* Spam
* Protections
* Firewall
* Spam filter
* Proxy
* Antivirus, antispyware
* Public/Private Keys System
* Reference: https://medium.com/@RiverChan/%E5%9F%BA%E7%A4%8E%E5%AF%86%E7%A2%BC%E5%AD%B8-%E5%B0%8D%E7%A8%B1%E5%BC%8F%E8%88%87%E9%9D%9E%E5%B0%8D%E7%A8%B1%E5%BC%8F%E5%8A%A0%E5%AF%86%E6%8A%80%E8%A1%93-de25fd5fa537
* SSL(Secure Scoket Layer)
* Application: sftp, https, ssh
* To prevent fishing, a certificate authority is needed, to ensure to correctivity of a key.
## RSA Algorithm
Reference: https://ithelp.ithome.com.tw/articles/10250721
* Generation
* Encryption

* Decryption