Understanding Networks
- Network Models
- What is a model?
- Models are used to represent how networks function.
- There are 2 very popular network models:
- the OSI 7-Layer model, &
- the TCP/IP model
OSI Model vs. TCP/IP Model
- OSI Model (older; more detailed)
- 7- Application
- 6- Presentation
- 5- Session
- 4- Transport
- 3- Network
- 2- Data Link
- 1- Physical
- TCP/IP Model (more modern; not as detailed/complicated/more simplified)
- 4- Application
- 3- Transport
- 2- Internet
- 1- Network Interface (Link)
- Notes: The OSI model is being supplanted with the TCP/IP model. The TCP Internet layer most closely matches the OSI Network layer.
- Meet The Frame (Packets)
- Devices on a network send & receive data in discreet chunks called frames (or packets).
- Frames are a maximum of 1500 bytes in size.
- Frames are created & destroyed inside the network interface card (NIC).
- The MAC Address (Media Access Control)
- A MAC address is a unique 48-bit identifier for a NIC.
- A MAC address is 12 numbers broken up into 6 pairs.
- The physical address is the MAC address. (OEM & Unique ID)
- ex: xx:xx:xx:xx:xx:xx
- OEM number is the 1st 3 pairs in sequential order. (OEM–Original Equipment Manufacturer)
- Unique Individual NIC Card Number aka ‘Unique ID’ is the last 3 pairs of the MAC address.
- Every NIC in existence has a unique MAC address!
- The MAC address is applied to the frame to ensure that it gets to the right place.
- A “to” (destination) MAC address & a “from” (source) MAC address.
- CRC (cyclic redundancy check)–used to verify the data is good. (If it’s bad data, then it knows to resend it.)
- Frames have destination & source MAC addresses.
- NICs use MAC addresses to decide whether or not to process a frame.
- A ‘hub’ is like a repeater; it takes a signal (data) that comes in from one part & then repeats it out to the other ports.
- The frame payload does not identify the destination. (This is where a MAC address comes into play!)
- Type ‘ipconfig’ (Windows) or ‘ifconfig’ (macOS) to see in the terminal.
- Broadcast vs. Unicast
- Unicast transmission is addressed to a single device on a network.
- A broadcast transmission is sent to every device in a broadcast domain.
- A broadcast address looks like this: FF-FF-FF-FF-FF-FF (Yes, all ‘F’s.)
- Broadcast domain is the area, or reach of computers that are able to receive the broadcast message.
- Usually all the computers on the hub.
- Introduction to IP Addressing
- (Note: IP addressing falls under a category known as ‘logical addressing’.)
- An IPv4 address looks like this: 31.44.17.231 or
- An IPv6 address looks like this: 2001::OD8B8:FE01::
- A router connects multiple local area networks.
- The IP packet within the frame never changes. (Note: Packets sit within frames.)
- Packets are always encapsulated in some type of frame; they don’t travel alone.
- A routing table is built into every router in the universe.
- The routing table specifies where to send/forward/route data.
- The frames can change during the journey from source to destination, BUT the IP packet remains the same!
- Packets & Ports
- Port numbers help direct packet traffic between the source & destination.
- Packets have sequence numbers so the network software can reassemble the file correctly.
- TCP is connection-oriented, UDP is connectionless.
- Port numbers are unique to individual applications that are used all over the Internet. (ex: port 80 is used to define a web page)
- Port range is 0-65,535.
- The first 1,024 port address numbers are called “well-known ports”. They are reserved.
- TCP is a connection-oriented conversation between 2 computers to make sure the data arrives whole, complete & in order. To do this it also uses:
- Sequencing Number–allows for reassembly
- Acknowledgement Number–acknowledges receipt of data. (UDP does NOT verify receipt of data.
- TCP is the overwhelmingly dominant protocol on the Internet.
