CS Resources

Computer Science Resources and Links from Avi Parshan

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Intro to Computer Communications

TCP/IP Model

  1. Application

    • Message
  2. Transport

    • Segment
  3. Network

    • Datagram
  4. Link

    • Frame
  5. Physical

OSI Model takes the Application layer and splits it to 3 different “layers”:

  • Application

  • Presentation

  • Session

8 Bits = 1 Byte


* 32 bit address = 4 bytes 

* 8 bits.8 bits.8 bits.8 bits In binary

* Each section can get values from 0-255 = 256 total values

* Network Address = set all host bits to 0 

* Broadcast address = set all host bits to 1 

* number of available host addresses - need to subtract 2 from # of available addresses (cause of net address and broadcast address) 

Packet transmission delay: L (bits) / R (bits/sec)

Propagation delay:

*  d(length of physical link)/s (prop speed)

*  Often times use speed of light 3*10^8 for fiber 

*  For copper use 2 * 10^8 

*  Still quite fast but will never get to speed of light 


  • Send a request via MAC to broadcast and ask for someone’s IP

  • MAC broadcast is all bits set to 1, which in HEX will result in an address with lot of F’s

  • It is a layer 2-ish protocol


  • time to live - 1 byte - so can be from 0 to 255

  • Measured in hops not seconds or any other time unit

  • Start with X for TTL, Every router we pass, TTL = TTL-1

  • Once we get to 0, packet gets dropped and often we get an ICMP response telling us that happened



    • As we need a destination port and 3 way handshake with someone

TCP Socket:

  • Needs all 4 of these: SRC IP, SRC PORT, DEST IP, DEST PORT

  • Socket is two-way like a pipe with opening on both sides (src - dest)

Broadcast packet never gets past the router (gets dropped), otherwise many more computers than anticipated will get these packets

Switch - Internal to a network

Forwarding table - Where to send packet to


  • Sits between your network and the internet (more or less)

  • Converts your internal IP to a public one via Network Address Translation


  • Wrapped in an IP packet

  • If it is an error report, you will get IP header - ICMP header - IP header

    • 2nd ip header is a minimum of first 8 bytes of the packet that caused the error
    • Can and may be more bytes of previous packet
  • Layer 3ish protocol

https://gaia.cs.umass.edu/kurose_ross/interactive/ (practice quizzes)


Updated on May 4, 2024