# A+ 220-1002: Day 11: LANs: Ethernet & WiFi

## Local Area Networks (LANs)

### The TCP/IP Model and the OSI Model

-are 32 bits long

-are expressed in Decimal (Base 10, not Hexadecimal like MAC addresses)

-are grouped in four groups separated by dots:

192.168.1.25

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:

10.x.x.x

or as long as:

192.168.254.x

…where x is the space for 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.

#### Virtual LANs (VLANs)

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:

169.254.x.x

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.

-Strictly for LAN communication

Neighbor Discovery Protocol replaces arp

‘How To “arp -a” In IPv6’
https://www.practicalcodeuse.com/how-to-arp-a-in-ipv6

https://oswalt.dev/2011/10/neighbor-solicitation-ipv6s-replacement-for-arp/

IPv6 ‘arp’ For Windows:

`netsh int ipv6 show neigh`

IPv6 ‘arp’ For Linux:

`ip -6 neigh`

-which is actually FE80:0000:0000:0000 (64 bits)

-followed by the 64-bit host ID

-for example:

fe80::86a:f9d:91ef:4a24

#### Broadcast vs. Multicast

fe02::02   # the All Routers multicast address

### Network Services

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.

### Network Commands

```arp   #  (Linux and Windows)

ping   # (Linux and Windows)

ipconfig # (Windows)

ifconfig # (old Linux command; deprecated)

ip # (new Linux command)

iwconfig # (Linux wifi)

nslookup  # (Linux and Windows)

dig # (Linux native, Windows installable)

traceroute # (Linux)

tracert # (Windows)

net  #  (Windows)

netstat # (Windows and Linux)

nbtstat # (Windows)

netsh  #  (Windows)

```

### Wifi

CompTIA says there are two modes of Wifi:

• Infrastructure Mode
• Ad Hoc Mode

There’s actually a difference between peer-to-peer and ad hoc, but not as far as this test goes.

Ad Hoc mode is disabled by default in Windows 8+, but can be enabled at the command line.

Standard Wifi Version Frequency Encryption Speed Beams Multiplexing
802.11a WiFi 1 5 GHz 54 Mbps 1
802.11b WiFi 2 2.4 GHz WEP 11 Mbps 1
802.11g WiFi 3 2.4 GHz WPA 54 Mbps 1
802.11n WiFi 4 2.4 GHz, 5 GHz WPA2 – PSK, Enterprise ~300 Mbps 4 MIMO
802.11ac WiFi 5 5 GHz WPA3 – PSK, Enterprise ~300 Mbps 8 MU-MIMO, OFDM
802.11ax WiFi 6 2.4 GHz, 5 GHz WPA3 – PSK, Enterprise ~300 Mbps 8 MIMOA, OFDMA
802.11ax WiFi 6e 6 GHz WPA3 – PSK, Enterprise ~300 Mbps 8 MIMOA, OFDMA

## Textbook Time

Chapters 19 and 20