Layer 1 – Physical

This is where the electrical bits are generated. Either for wired, wireless, or fiber connections. These are the electrical pulses that create the 1’s and 0’s. 

  • Wiring
  • Fiber
  • Network Cards
  • Hubs
  • Ethernet (IEEE 802.3), Token Ring, RS-232, FDDI, and others

Layer 2 – Data-link

  • Ethernet frames
  • Network card and drivers
  • PPP
  • Switches
  • Bridge
  • MAC addresses
  • Layer 2 network maps include the physical layer 1 port
  • Translates data frames into bits for Layer 1 processing
  • PPP, IEEE 802.2

Layer 3- Network

  • Involves subnets and routers
  • IP Addresses
  • Packets
  • ARP
  • ICMP
  • IPv4, IPv6, ARP, ICMP

Layer 4 – Transport

  • Packets: RAW, TCP (segments), UDP (datagrams)
  • end-to-end management of the message
  • responsible for getting the entire message (fragmentation, out of order packets, etc)

Layer 7 – Application

  • Specific format for the program
  • NFS, NIS, DNS, LDAP, telnetftprloginrshrcp, RIP, RDISC, SNMP, and others
  • Application layer protocols


  1. The OS is notified by the network card that there’s data (Layer 1 & 2)
  2. The network card driver unwraps the Layer 2 frame and hopefully exposes a Layer 3 packet
  3. If the OS decides to keep the packet, it unwraps it into a Layer 4 packet.
  4. Depending on what the packet is (TCP, UDP, etc) it sends it to the proper subsystem.
  5. The subsystem will then unwrap the Layer 7 data and send it to the proper application with the designated port.
  6. Going the opposite way, routers will peek inside the packets to see where it’s destined for. If it is destined for a system on the subnet, the OS will simply send an ARP instead of going through the router.
OSITCP/IPProtocol Data UnitEncapsulationDevicesProtocol Suite
7ApplicationApplicationDataData (Unsplit)Layer 7 FirewallHTTP, DNS, DHCP, FTP, Telnet, SSH, SMTP, POP, IMAP, NTP, SNMP, TLS/SSL, BGP, RIP, SIP
4TransportTransportSegmentsAdd transport headerLayer 4 FirewallTCP, UDP
3NetworkInternetPacketsAdd network headerRouterIPv4, IPv6, ICMP, ICMPv6, IPSec, OSPF, EIGRP
2Data LinkNetwork AccessFramesAdd frame header and trailerSwitch, NIC, Wireless APMAC, ARP, Ethernet 802.3, CDP, LLPDP, HDLC, PPP, DSL, L2TP, 802.11, SONET/SDH
1Physical BitsHub, NIC, Wireless AP