As a programmer, it's impossible to avoid dealing with the Internet. Today, our mobile phones and computers are so deeply integrated with the network that they would be nearly useless without it. The role of this article is to help those who are not network professionals grasp the essentials of networking in the shortest time possible.
Let’s start by understanding some commonly known but often misunderstood technical terms.
**Internet**
The Internet is the largest network in the world today, often referred to as a "network of networks." It is essentially a vast interconnected system of all global networks.
**Composition of the Internet:**
- **Edge part:** Hosts (devices like computers and smartphones)
- **Core:** A large number of networks and routers connecting these networks (not your home router)
**Ethernet**
Ethernet is the most widely used local area network (LAN) communication protocol. Ethernet frames are transmitted over Ethernet. Since Ethernet allows only one computer to send data at a time, there must be a detection mechanism, which is the CSMA/CD protocol:
- **Multi-point access:** Multiple computers are connected to one bus in a multipoint access mode.
- **Carrier monitoring:** Each station must constantly detect the channel regardless of whether it is transmitting or not.
- **Collision detection:** Listening while sending.
**OSI Model**
The Open System Interconnection Basic Reference Model defines how data should be transmitted between systems. Any two systems can communicate if they follow the OSI standard. The OSI model has seven layers, while TCP/IP uses a four-layer architecture. In practice, the five-layer model is often used when learning about computer networks: physical layer, data link layer, network layer, transport layer, and application layer.
**Protocol Architecture**
**Physical Layer**
In the world of computers, everything is represented in 0s and 1s. However, these bits cannot be transmitted directly through physical media; they need to be converted into optical or electrical signals. This layer is responsible for converting bit streams (0101) into photoelectric signals.
Without the physical layer, there would be no Internet, no data sharing, because data could not flow through the network.
**Data Link Layer**
At this layer, data is no longer sent as bit streams but is divided into frames and then transmitted.
**MAC Address**
Also known as the hardware address of a computer, the MAC address is a 48-bit address stored on the network card's ROM. It uniquely identifies a computer because it is unique worldwide.
**Packet Switching**
Since data is divided into frames, and different links have different maximum frame lengths (MTUs), frames that exceed the MTU must be fragmented. For example, a truck can carry 5 tons of cargo at once, but if a road can only support 2 tons, you need to make three trips.
**Bridge**
A bridge operates at the data link layer and forwards and filters received frames based on the destination MAC address.
**Ethernet Switch**
An Ethernet switch is essentially a multi-interface bridge. Each interface of an Ethernet switch is directly connected to a single host or another hub, making it easy to implement VLANs (Virtual Local Area Networks).
**Ethernet MAC Frame**
The format of the MAC frame includes:
- Destination address: 48-bit MAC address of the receiver
- Source address: 48-bit MAC address of the sender
- Type field: Indicates the protocol used on the upper layer (e.g., 0x0800 for IP datagrams)
**Network Layer**
If only the data link layer is present, data can only be transmitted on the same link and cannot be transmitted across different links. With the network layer, data can be transmitted across different data links.
**IP Address**
An IP address, also known as a software address, is stored in a computer's memory. IPv4 addresses are 32 bits long, while IPv6 addresses are 128 bits.
**IP Address vs. MAC Address**
IP addresses are used above the network layer, while MAC addresses are used below the data link layer. IP addresses are logical, while MAC addresses are physical. The source and destination addresses in an IP packet header remain unchanged during transmission, while those in a MAC frame change at each router.
**IP Address Classification**
An IP address is composed of a network number and a host number:
- Class A: 0.0.0.0 to 127.0.0.0
- Class B: 128.0.0.0 to 191.255.0.0
- Class C: 192.0.0.0 to 223.255.255.0
**Subnetting**
After subnetting, an IP address is composed of a network number, a subnet number, and a host number. For example, a company with a Class B IP address 145.13.0.0 can use internal subnets like 145.13.3.0, 145.13.7.0, and 145.13.21.0, while externally it still appears as 145.13.0.0.
**Subnet Mask**
A subnet mask typically consists of a string of 1s followed by a string of 0s. Regardless of whether the network is subnetted, the subnet mask and IP address are bitwise ANDed to determine the network address.
**Supernetting**
Supernetting involves combining multiple networks into a single block using a network prefix instead of class-based addressing. CIDR (Classless Inter-Domain Routing) allows for more flexible routing.
**IP Datagram**
At the network layer, data is transmitted in the form of IP datagrams (IP packets). These include a header and a data section.
**IP Datagram Header**
The first 20 bytes of the header are fixed, while the last 4 bytes are optional. Key fields include:
- Version: IPv4 or IPv6
- Header Length: Specifies the length of the header
- Total Length: The total size of the IP datagram
- Identification: Used for fragmentation
- Flags: Indicates whether the datagram can be fragmented
- Fragment Offset: Specifies the position of the fragment within the original datagram
- Time to Live (TTL): Limits the number of hops a datagram can take
- Protocol: Indicates the protocol used to carry the data
- Checksum: Verifies the integrity of the header
**IP Layer Forwarding**
Each router maintains a routing table containing entries like (destination network address, next hop address). When forwarding packets, the router checks the destination address and determines the next hop accordingly.
**Virtual Private Network (VPN)**
Routers on the Internet do not forward datagrams with private IP addresses. There are three ranges of private IP addresses:
- 10.0.0.0 to 10.255.255.255
- 172.16.0.0 to 172.31.255.255
- 192.168.0.0 to 192.168.255.255
To connect two private networks, such as a company's branch offices, a virtual private network (VPN) can be used over the public Internet.
**Network Address Translation (NAT)**
Multiple hosts in private networks share the IP address of a NAT router. When hosts send and receive IP datagrams, they go through the NAT router for address translation.
**ARP Protocol**
The Address Resolution Protocol (ARP) maps IP addresses to MAC addresses on the same LAN. Each host maintains an ARP cache with mappings of IP addresses to MAC addresses.
**Transport Layer**
This layer is crucial because the data link and network layers provide best-effort delivery, but the transport layer ensures reliable transmission using protocols like TCP and UDP.
**User Datagram Protocol (UDP)**
Key features of UDP:
- Connectionless
- Best-effort delivery
- Message-oriented
- No congestion control
- Supports various communication modes
- Small header overhead (only 8 bytes)
**Transmission Control Protocol (TCP)**
TCP is a connection-oriented, reliable, full-duplex, byte-oriented transport protocol. It ensures data is delivered correctly and in order.
**TCP Header**
The TCP segment header includes fields like source port, destination port, sequence number, acknowledgment number, data offset, flags, window size, checksum, urgent pointer, and options.
**Window and Flow Control**
The sliding window mechanism improves channel utilization by allowing multiple segments to be sent before waiting for acknowledgments. Flow control manages the rate at which data is sent to prevent overwhelming the receiver.
**Congestion Control**
TCP uses congestion windows (cwnd) and slow start algorithms to manage network congestion. It also implements fast retransmission and fast recovery mechanisms to handle lost packets efficiently.
**TCP Three-Way Handshake**
TCP establishes a connection through a three-step process: SYN, SYN-ACK, and ACK. This ensures both parties confirm the connection is ready for data transfer.
**TCP Four-Way Finishing**
Closing a TCP connection requires four steps: FIN from the sender, ACK from the receiver, FIN from the receiver, and ACK from the sender. This ensures both sides confirm the connection is fully closed.
**Application Layer**
The application layer includes protocols like HTTP, FTP, and DNS. These protocols enable user-facing services such as web browsing, file transfer, and domain name resolution.
**Domain Name System (DNS)**
DNS translates domain names into IP addresses. It involves root servers, top-level domain servers, authoritative servers, and local servers. Queries are typically recursive or iterative.
**URL Format**
A URL has the structure: `protocol://host:port/path`. Common examples include `http://example.com` and `https://example.com`.
**HTTP Protocol**
HTTP is transaction-oriented and connectionless. HTTP/1.1 introduced persistent connections to improve efficiency. GET and POST methods are used for retrieving and submitting data, respectively.
**Cookie and Session**
Cookies store user session information on the client side, while sessions store it on the server. Both are used to maintain state between requests.
**HTTPS**
HTTPS adds a layer of SSL/TLS encryption to HTTP, ensuring secure communication by preventing impersonation, tampering, and eavesdropping.
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