OSI Model & Networking

Transport (layer 4)

Network (layer 3)

Data Link (layer 2)

Session (layer 5)

Video calls and games often use UDP; web pages and email use TCP.

IPv4 allows about 4.3 billion addresses.

It translates domain names (like studyfox.pro) into IP addresses.

443

A switch forwards frames inside a local network by MAC address (layer 2). A router forwards packets between networks by IP address (layer 3).

MAC address

1. Physical 2. Data Link 3. Network 4. Transport 5. Session 6. Presentation 7. Application

Layer 1: bits Layer 2: frames Layer 3: packets Layer 4: segments (TCP) or datagrams (UDP) Layers 5–7: data

Presentation (layer 6)

HTTP/HTTPS, DNS, SMTP, FTP, SSH, DHCP

Layer 1: hubs, repeaters, cables Layer 2: switches, bridges, network cards Layer 3: routers (and layer 3 switches)

A hub repeats every incoming signal out of all ports (one shared collision domain). A switch learns MAC addresses and sends each frame only to the port where the destination lives.

Application (OSI 5–7) Transport (OSI 4) Internet (OSI 3) Network Access / Link (OSI 1–2)

It sets up a connection and synchronizes sequence numbers before any data is sent.

80

22

53

25

3389

FTP: 20 (data) and 21 (control). Telnet: 23.

It automatically gives a device an IP address, subnet mask, default gateway and DNS server. Steps (DORA): Discover, Offer, Request, Acknowledge.

It finds the MAC address that belongs to a known IP address on the local network.

10.0.0.0/8 172.16.0.0/12 192.168.0.0/16

It rewrites private IP addresses into a public one (and back) at the router, letting many devices share a single public IPv4 address.

Subnet mask 255.255.255.0

254

127.0.0.1 in IPv4 (localhost) and ::1 in IPv6. Traffic sent there never leaves the machine.

The router a device sends traffic to when the destination is outside its own subnet.

Unicast: one sender to one receiver. Broadcast: one to every device on the local network. Multicast: one to a subscribed group.

ICMP (Internet Control Message Protocol), at the network layer.

0–1023. Registered ports are 1024–49151, and dynamic (ephemeral) ports are 49152–65535.

A LAN (local area network) covers a small area such as a home or office. A WAN (wide area network) connects networks across large distances; the internet is the largest WAN.

Star: every device connects to a central switch (most common LAN layout). Bus: all devices share one cable. Ring: each device connects to two neighbors in a loop. Mesh: devices have multiple paths to each other (most fault tolerant).