# <center><i class="fa fa-edit"></i> Network Fundamentals: Networking </center>
###### tags: `Internship`
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**Goal:**
To gain a basic understanding of the components behind network technologies. Focus on vocabulary and systems overview.
- [x] How Networks Work

**Resources:**
[Juniper Online Learning Modules](https://learningportal.juniper.net/juniper/user_activity_info.aspx?id=769)
[Pre-Internship Notes by Jessica Chen](https://hackmd.io/@j-chen/SyfRATOmD)
[Daily Report by Lin Yen-Ting](https://hackmd.io/@8KbRc796SnuYA2Dvsvk_BA/SJU_CawIL)
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## Module 1: How Networks Work

### Overview
- Network: conduit that connects two or more computers or other devices
- Local Area Network (LAN): any group of computers on a single, geographically limited network
- wired (switch)
- wireless (access point)
- Wide Area Network (WAN): a network that connects LANs over large geographic distrances
- Open Systems Interconnection Reference Model (OSI): Identifies seven layers (network stack) of functionality computers use to communicate over a network
- Does not define specific procedures or protocols; theoretical in nature
- Sending Computer: data flows downward. Protocols are added as headers or footers before passed onto the next layer (encapsulation)
- Receiving Computer: data flows upward. Protocols are examined and removed layer by layer (deencapsulation)
- Transmission Control Protocol/Internet Protocol Reference Model (TCP/IP)
- Simple, four layer model
- Defines specific protocols at each layer
- Mergers top three layers of OSI into Application Layer
- Transport and Network layers map
- Merges lower two of OSI into Network Access Layer
- Five-Layer Model
- Combo of OSI and TCP/IP
- Unmerges Network Access Layer back into Data Link and Physical

### Traversing the Five-Layer Model
- Application Layer: data processed using protocols on sending computer
- How data should be encoded, compressed, or encrpyted
- How sessions should be managed
- Commonly used Application Layer Protocols
- Simple Mail Transport Protocol (SMTP): most popular for sending email
- Hypertext Transder Protocol (HTTP): communications; Internet to WWW
- File Transfer Protol (FTP): network; computer to another computer (i.e. the Internet)
- Domain Name System (DNS): associates various info with domain names; serves as phone book for internet; hostnames to IP addresses
- Letter with no envelope and only sender info
- Transport Layer: specifies Application Layer Protocol on receiving computer
- Port: Unique numberical indetifier for the protocol
- address of recipient
- multiple programs at same port analogous to multiple people living at same address
- Destination port number: specifies which Application Layer protocol to be used on receiving computer so incoming data can find the right program
- name of recipient
- Source port number: identifies source
- putting return name
- Transport Layer Protocols
- User Datagram Protocol (UDP): simple and fast "best effort" delivery service; commonly used for short messages and time-sensitive data; allows for packet loss with high impact on performance (i.e. DNS, online gaming, Voice over, streaming video)
- Transmission Control Protocol (TCP): provides delivery notifcation, error checking, recovery procedures; more robust than UDP
- cuts data into segments to ensure steady flow of data regardless of differences in bandwith
- 
- Network Layer: recevies data segments and adds a header to create a packet
- Header contains destination and source IP address; identifies a Transport layer protocol
- add recipient name and return address
- ![] (https://i.imgur.com/dr3iTEI.png)
- Data Link Layer: receives packets and adds its won header to each packet to create a frame
- Headers includes another address (Layer 2 address, physical address, MAC address)
- Checksum: data integrity check method added to trailer at end of frame
- Converts data into ones and zeroes for digital communication
- post office sorting containers
- 
- Physical Layer: specifications define characteristics (i.e. cabling specs, voltage levels, physical data rates, max transmission distances, physical connectors)
- carries of letters between post offices
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### Module 1 Summary
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