What is a Network?
Multiple computers that are connected together and share information
What is the Internet?
The Internet is a network of networks
What is a WAN?
A wide area network that extends over a large geographic area
What is a LAN?
A local area network that covers a small geographic area
What is a PAN?
A personal area network that covers a short-range around a person
What are the two main layered network models?
- OSI model
- TCP/IP Model
What are the 4 layers in the TCP/IP model?
- Application Layer
- Transport Layer
- Internet Layer (Network Layer)
- Network Access Layer (Link Layer)
Why use a layered model?
- Abstraction
- Irrespective of underlying hardware and topology, applications and host can talk to each other
What is the End-to-End Concept?
- The network is responsible for providing best-effort connections and should be essentially transparent
- End hosts are responsible for reliability and security
What is Layer Encapsulation?
- Each layer in the TCP/IP stack adds it’s own header to the data
- The becomes the payload for the next layer
What does the Network Access Layer do?
- Deals with the local link that a host is connected to
- Each host has a link-unique address
- e.g. encapsulation of a packet in an ethernet frame
What is LAN defined by?
The IEEE 802 family of standard
What are the basic functions of the Internet Layer?
- Handles next-hop routing
- Provides unique addressing
- Passes a received packet’s payload to the transport layer
- Facilitates connection of different types of network
- The Internet layer only provides a “best-effort” packet delivery
What are the main protocols of the Internet Layer?
- IPv6
- IPv4
- ICMPv6
- ICMP
- IPSEC
What is IPv4?
- Internet Protocol version 4
- Each node has a unique 32-bit IP address
- Written in octet-grouped dotted decimal notation (152.78.65.112)
- Variable length header, minimum of 20 bytes
What is IPv4 Exhausion?
A 32-bit address space only allows for 4.3 billion addresses so they will eventually run out which has been anticipated since the 1980s
What is NAT and NAPT?
- Network Address (and port) Translation is a way to share one IPv4 address between multiple computers
- A home router shares a public IPv4 address with multiple devices having a private IPv4 address
What are the issues with NAT and NAPT?
- NAPT breaks the end-to-end principle & some protocols
- Gives a false sense of security - NAT is not a replacement for a properly configured firewall
- Even with NAT, we still have IPv4 exhaustion
What is IPv6?
- Internet Protocol version 6
- Each node has a unique 128-bit IP address, written in colon-delimited hexadecimal e.g. 2001:630:d0:f111:e07a:b1fa:68a1:80eb
- 40-byte header
What is a subnet?
A logical subdivision of a network
What is routing?
- Occurs where there is a change in IP address spaces at the Internet Layer
- Each router has an IP address in each address space it routes between
- Routers can also have other functions: firewalling, dns, dhcp etc
What does the Transport layer do?
Provides host-to-host communication
- Using two main protocols, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)
What are the characteristics of TCP?
- Connection oriented
- Handles ACK & retransmissions
- Guaranteed that it arrives and in the correct order
- The header is 20-bytes minimum
- It is good for applications that need high reliability
- Some example protocols are: HTTP(S), FTP, SMTP, SSH
What are the characteristics of UDP?
- Connectionless: “Fire and Forget”
- Application needs to handle ACK & retransmissions if needed
- No guarantee that data is received in the order sent
- The header is 8-bytes
- It is good for applications that need fast and efficient transmission
- Example protocols: DHCP. TFTP, RIP
