Computer Network

lastmod 2026-09-15created 2014-01-1166 edits

Network administrator
- brctl
- tracert or traceroute
- lsof
- route
- nslookup
- ipcalc
- Wireshark
- libpcap
- nmap
- Moloch
- wrk
Network management
- Distributed Management Task Force (DMTF)
  - Desktop Management Interface (DMI)
    - dmidecode (Linux)
    - wmic (Windows)
Computer network
- Proxy
  - Tor
- Load balancing
- Reverse proxy (Forward proxy; Edge Router)
  - Traefik
- Virtual LAN (VLAN)
  - OpenVSwitch
- Network virtualization (Virtual network)
  - DaoliNet
  - Vrnetlab
  - Calico
- Router
- Virtual private network (VPN)
  - OpenVPN
- Network topology
  - Local Area Network (LAN)
    - IEEE 802.11
  - Metropolitan area network (MAN)
  - Wide area network (WAN)
- Network Address Translation (NAT)
- CIDR
- Peer-to-peer (P2P)
  - Torrent
  - Tribler
- Software-defined networking (SDN)
  - OpenFlow
Internet
- World Wide Web (WWW)
  - Web search engine
    - Web crawler
- Mobile Web
~~Server~~
(covered on a separate page)
Infrastructure
- Terraform
Terms
- Promiscuous mode
- Network congestion (network congestion collapse)
- Masking Latency
Data transmission
- Communication protocol
  - Stateless protocol
  - Stateful protocol (State)

A computer network is a set of distributed computers connected by a communication network.

Commands between calculating machines and early computers were mostly relayed directly by humans. In September 1940, George Stibitz used a teletype machine to send commands to the Complex Number Calculator in New York and receive the results. Connecting output systems such as teletypes to computers was a matter of great interest to J.C.R. Licklider in 1962, when he developed a working group called the so-called 'interplanetary network'. This 'interplanetary network' later became the predecessor of ARPANET.

Detecting IP conflicts

See arp-scan#Detecting IP conflicts.

Communicating between two or more PCs over a direct connection

Wangjaengi's Computer Journey : Naver Blog - Connecting two PCs directly - wired/wireless LAN card setup ¹

How to connect a network or the Internet between PCs without a router:

Check the name of each network interface (Ethernet port) connected to the server. Ethernet interfaces are usually shown as "eth0," "eth1," "eth2," "eth3," and so on. You can check with the following command.

ifconfig -a

Configure a different subnet for each Ethernet port. For example, you can set eth0 to 192.168.1.1/24, eth1 to 192.168.2.1/24, eth2 to 192.168.3.1/24, and eth3 to 192.168.4.1/24.

sudo ifconfig eth0 192.168.1.1/24 up
sudo ifconfig eth1 192.168.2.1/24 up
sudo ifconfig eth2 192.168.3.1/24 up
sudo ifconfig eth3 192.168.4.1/24 up

You need to set up routing for direct communication through each Ethernet port. Add routing rules so that traffic is sent to the correct Ethernet port. The subnets must be different from each other

sudo ip route add 192.168.2.0/24 dev eth1
sudo ip route add 192.168.3.0/24 dev eth2
sudo ip route add 192.168.4.0/24 dev eth3

(Optional) If needed, you can set firewall rules to allow or block specific ports or services. For example, you can set firewall rules with iptables.

sudo iptables -A INPUT -i eth1 -j ACCEPT
sudo iptables -A INPUT -i eth2 -j ACCEPT
sudo iptables -A INPUT -i eth3 -j ACCEPT

Using a Linux server as a router

See net.ipv4.ip_forward for details.

Promiscuous mode

eth0: Promiscuous mode enabled

When the TCP/IP protocol was designed, datagrams were meant to be sent without any encryption at all. So if you collect the packets moving across the network and reassemble them in the right order, you can get the original data.. This method is called sniffing. It means to sniff around..

That means someone could intercept your ID and password when you connect from a remote host, right? On Ethernet, the source host scatters data to multiple hosts. Then.. each host just lets the data pass if it isn't its own, and if it is its own, it receives it and does something with it.

By default, a system is set to receive only its own data. But when you run a sniffer, your system's interface opens up and accepts anything.. This mode is called promiscuous mode. If your system is running in promiscuous mode, you might suspect that a sniffer is running....

When problems occur because two interfaces have the same subnet

How to connect two network interfaces on the same subnet? - Red Hat Customer Portal

Root cause:

When there are two interfaces on the same subnet, there is no guarantee which interface will be used to send traffic, and the system accepts traffic for both IPs on both interfaces.
This is because on Linux, IP addresses belong to the host and are not tied to interfaces.
If you ping with -I DEV to use a given interface, there is no guarantee that the reply packets (if any) will come back on the same interface, so -I DEV may not work.

Network layer models

The Internet is based on the TCP/IP model, which first developed among private operators and became the backbone of the major Internet networks. This model can be compared, as shown below, with the OSI 7-layer model established earlier by an international standards organization.

OSI model	Internet model (Internet protocol suite or TCP/IP model)
Application layer	Application layer
Presentation layer
Session layer
Transport layer	Transport layer
Network layer	Internet Layer
Data link layer	Network Access Layer
Physical layer	Network Access Layer