tags:: #note/boat #note/develop #on/networking # Networking Fundamentals A **computer network** allows nodes to share resources by communicating with each other. ## Networking Devices ### Clients A **client** is a device that uses the resources and services provided by [[Networking Fundamentals#Servers|servers]]. ### Servers A **server** is a device that provides resources and services for [[Networking Fundamentals#Clients|clients]]. ### Switches A **switch** is used to provide connectivity within a [[Local Area Network|Local Area Network (LAN)]]. #### Characteristics of Switches - Have many network interfaces/ports for end hosts to connect to - Provide connectivity to hosts within the same LAN - Do not provide connectivity between LANs or over the internet ### Routers A **router** is used to provide connectivity between LANs. #### Characteristics of Routers - Have fewer interfaces/ports than switches - Used to send data over the internet ### Firewalls **Firewalls** are security devices that control traffic entering and exiting a network. They must be configured with rules to control which traffic is allowed and which is blocked. **Host-based firewalls** are software applications that filter traffic on a host machine. #### Characteristics of Firewalls - Monitor and control network traffic based on configured rules - Can be places inside or outside the network - "Next Generation Firewalls" include modern and more advanced filtering capabilities ## Networking Models Networking models provide a structure for networking protocols and standards. **Protocol**: set of rules defining how network devices and software should work ### OSI Model - Not in use today, but still referred to often - OSI stands for "Open Systems Interconnection" - Created by ISO in the late 70s/early 80s - Categorizes different functions in a network - Functions are divided into 7 "Layers" that work together to make the network work - Data is encapsulated as it moves down the stack, then de-encapsulated as it moves back up - Network engineers don't usually work with the top 3 layers (5-7), but devs do - Top 3 layers prepare the data, bottom 4 layers do the actual work of sending it over the network - **Protocol Data Units (PDUs)**: name to reference data, segments, packets, and frames | **Layer** | **Name** | **Description** | | ---- | ---- | ---- | | 7<br> | Application | Interacts with software applications | | 6 | Presentation | Translates between application and network formats | | 5 | Session | Controls sessions between communicating hosts | | 4 | Transport | Segments and reassembles data for communications between hosts | | 3 | Network | Provides logical addressing and connectivity between hosts | | 2 | Data Link | Provides node-to-node connectivity | | 1 | Physical | Defines physical characteristics of the medium used to transfer data | #### Layer 7 - Application - Closest to the end user - Interacts with software applications - HTTP and HTTPS are Layer 7 protocols - Doesn't include the application itself - Functions: - Identifying communication partners - Synchronizing communication #### Layer 6 - Presentation - Data in application layer is in "application format", so it needs to be translated to a network format to be sent over the network - Presentation layer does the translating - Example: encryption and decryption - Also translates between different Application-Layer formats #### Layer 5 - Session - Manages connections between the local application and the remote application #### Layer 4 - Transport - Breaks large pieces of data into smaller segments that are easier to send over the network - Smaller segments are less likely to cause transmission problems if errors occur - Provides host-to-host communication - Appends a Layer 4 header to the end of the data - Combination of data and L4 header is called a **segment** #### Layer 3 - Network - Provides connectivity between hosts on different networks - Provides logical addressing (IP addresses) - Provides path selection between source and destination - Routers operate at layer 3 - Appends a Layer 3 header to the segment - Includes source and destination IP addresses - Combination of data, L4 header, and L3 header is called a **packet** #### Layer 2 - Data Link - Adds a L2 header and tailer to the packet, creating a **frame** - Defines how data is formatted for transmission over a physical medium - Detects and corrects errors from Physical Layer - Uses Layer 2 addressing, separate from Layer 3 - Switches operate at layer 2, look at destination address to know where to send data to #### Layer 1 - Physical - Defines physical characteristics of data transfer medium (ex, voltage levels, physical connectors, cable specs, etc.) - Digital bits converted to electrical or radio signals ### TCP/IP Suite - Conceptual model and set of communications protocols - Developed by DARPA - Actually used in modern networks - OSI layers 5-7 merged to be TCP/IP Layer 4 - Transport layer is the same between OSI and TCP/IP - Network layer of OSI maps to Internet layer of TCP/IP - OSI layers 1-2 map to TCP/IP Link layer - When people talk about layers, they usually mean OSI layers | **Layer** | **Name** | **Description** | | ---- | ---- | ---- | | 4 | Application | Combination of OSI layers 5-7 | | 3 | Transport | Same as OSI transport layer | | 2 | Internet | Same as OSI network layer | | 1 | Link | Combination of OSI layers 1-2 | ## References _Free CCNA | Network Devices | Day 1 | CCNA 200-301 Complete Course_. YouTube Video. Vol. 1. 63 vols. CCNA 200-301 Complete Course, 2019. [https://youtu.be/H8W9oMNSuwo?si=-hl3EvXq8JcOK6HP](https://youtu.be/H8W9oMNSuwo?si=-hl3EvXq8JcOK6HP). _Free CCNA | OSI Model & TCP/IP Suite | Day 3 | CCNA 200-301 Complete Course_. YouTube Video. Vol. 3. 63 vols. CCNA 200-301 Complete Course, 2019. [https://youtu.be/t-ai8JzhHuY?si=fPFljSbhkeEJODXh](https://youtu.be/t-ai8JzhHuY?si=fPFljSbhkeEJODXh).