Computer Networks Flashcards
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
What is TCP Flow Control?
- Prevents a fast sender overwhelming a slow receiver
- TCP uses a sliding window protocol to control the sending rate
- Sender should only send if the receiver indicates it has buffer space to accept data, the sliding window is the buffer
What is TCP Congestion Control?
- Reduces send rate to cope with network congestion
- Sender starts by sending small packets
- Sender increases the size of each subsequent packet until there is packet loss (Exponentially up to a threshold then additively)
- Sender restarts the cycle with a lower threshold
- Done because congestion leads to packets being dropped
What does the Application layer do?
- Software that uses the network
- Generally, programmers will use pre-made libraries but need to choose appropriate modes
What is ICMP/ICMPv6?
- Used for diagnostic and control purposes or generated response to errors in IP operations
- Used by Ping and Traceroute
What Name/Address Type is in the Application Layer?
Domain name
What Name/Address Type is in the Transport Layer?
Port Number
What Name/Address Type is in the Network Layer?
IP address
What Name/Address Type is in the Link Layer?
MAC Address
What does ARP and NDP do?
Address Resolution Protocol (IPv4) and Neighbour Discovery Protocol (IPv6) operate at the Link Layer and translate IP address to MAC addresses
What is the DNS?
- The Domain Name Service provides a way to map symbolic domain names to an IPv4 or IPv6 address
- Highly-reliable and resilient distributed service that operates at the application layer
What does a switch do?
- Used to connect multiple devices on one network segment
- Switches operate at the Network Access layer
- Switches only forward traffic between the ports it needs to
How can you get a working IPv6 on a IPv4?
A Tunnelbroker e.g. Hurricane Electrics but it will have high latency than a native IPv6 connection
How do you stay anonymous on the internet?
VPNs
The Onion Router
How do VPNs work
