- A+ Certification
- A+ 220-1001: Day 1: Intro, Resources & the Test
- A+ 220-1001: Day 2: CPUs
- A+ 220-1001: Day 3: RAM
- A+ 220-1001: Day 4: Firmware, Motherboards & Power Supplies
- A+ 220-1001: Day 5: Disks & Mass Storage
- A+ 220-1001: Day 6: Peripherals & PC Builds
- A+ 220-1001: Day 7: OS Operations, User Management & OS Maintenance
- A+ 220-1001: Day 8: Catch-up & Review
- A+ 220-1002: Day 9: The Command Line & OS Troubleshooting
- A+ 220-1002: Day 10: Displays & Networking Basics
- A+ 220-1002: Day 11: LANs: Ethernet & WiFi
- A+ 220-1002: Day 12: The Internet & Virtualization
- A+ 220-1002: Day 13: Portable & Mobile Computing
- A+ 220-1002: Day 14: Mobile Administration & Printing
- A+ 220-1002: Day 15: Security & Operations
- A+ 220-1002: Day 16: Review & Test Preparation
Local Area Networks (LANs)
The TCP/IP Model and the OSI Model
-are 32 bits long
-are expressed in Decimal (not Hexadecimal like MAC addresses)
-are grouped in four groups separated by dots:
Each group is called an “octet” because it consists of 8 bits.
8 bits can express any number from 0 – 255.
The magic 8-bit calculator:
There are 8 bits, each of which can be a 0 or a 1:
0 0 0 0 0 0 0 0
Each bit is worth:
128 64 32 16 8 4 2 1
Use this calculator to translate any number from 0 – 255.
Every IP address consists of two parts: the network number (the first part), and the host ID (the second part).
But the IP address doesn’t just divide in half. It can divide at any number. So the network number could be as short as:
or as long as:
…where x equals the host ID.
The subnet mask is a string of 1s, but translated to decimal, for instance 24 1s would be 255.255.255.0. This would mean that the first 24 bits are the network number, and the last 8 bits are for host IDs.
Most hosts get an IP address assigned to them by a DHCP server. But if that server is down, hosts won’t have a way to get onto the local network.
Microsoft solved this with a “fall-back” protocol: APIPA. It lets hosts self-assign an IP address in a special range:
So if you do an ipconfig and see a host has a 168.254.x.x IP address, that means it failed to get a DHCP assignment. This is a critical testing point.
DNS – Domain Name Service (matches domain names to IP addresses)
DHCP – Dynamic Host Control Protocol (issues IP address, subnet mask, default gateway and at least 1 DNS server).
TCP vs UDP
TCP communications do lots of error-checking to ensure perfect data transfer. There’s lots of overhead to doing this.
UDP communicates like a fire hose: a jet of information with no error-checking. It’s very fast but nothing is guaranteed.
ping ipconfig # (Windows) ifconfig # (old Linux command; deprecated) ip # (new Linux command) iwconfig # (Linux wifi) nslookup traceroute # Linux) tracert # (Windows)
Chapters 19 and 20