- [Instructor] It seems that
no matter how high on the ladder of success I climb,
I will always end up walking someone through
the same basic steps to find their IP, DNS servers,
perform a ping or traceroute, and verify their ARP entry.
The domain name system, or DNS,
is a means to refer to an IP address
via an easier-to-remember ASCII name.
If I want to reach my personal web server,
I can browse to it by memorizing the IP,
209.189.22.8152,
and typing it in, or I can simply browse to gregsolo.com.
In essence, a client device will probe
their configured DNS servers for resolution.
The DNS server will then pass back
the corresponding IP address,
and then the client can continue along their way.
I'll also check the cached entries
in the Address Resolution Protocol table.
When a device on the network wants to
move down the OSI model, it must do a lookup
to find out what MAC address at Layer 2 corresponds
to what IP address at Layer 3.
A device will send an ARP
into the appropriate Layer 2 network,
asking whoever owns the IP address in question
to kindly respond with its MAC address.
These entries are then cached in the ARP table.
The network admin's best friend is the Ping utility.
It utilizes Internet Control Message Protocol
messages for testing.
It sends out a special echo request packet
to an IP address, and times how long it takes
for the client to receive
and respond to the message with an echo reply.
The Traceroute utility also utilizes ICMP to determine
all of the router hops along the way
to a particular client, along with the latency to each hop.
In Windows 10, this is performed
by getting into the command prompt.
This is easily done, by clicking the Start button,
and typing CMD and hitting Enter.
From there, it's a simple ipconfig/all
to get detailed information.
From here, you can verify IP addressing,
MAC addressing, whether or not DHCP is enabled,
or being utilized, DNS servers,
and the configured default gateway.
I can also perform my basic troubleshooting,
like pinging my default gateway.
Ping 192.168.88.1.
If, for some reason, I can't reach it,
I can check that ARP is working correctly.
Arp -a.
If I can't ping the host, but ARP shows a MAC address,
then I know Layer 2 is most likely working.
As shown, everything is working fine there.
So I can perform some traceroutes
to check further network connectivity.
Tracert -d.8.8.8.8
I usually use the -d, so that it won't attempt
to DNS-resolve the hops, as this slows the process down.
For any Linux users, the process isn't wholly different.
Differing versions of Linux do things differently,
but I'll do a quick demonstration using CentOS 7.
When you using other flavors and versions,
Google is an amazing resource.
To view IP addressing,
I'll issue the ip addr command.
In this instance, entry two is my active network interface.
The ip route command
shows all routing entries in the kernel.
The default route, zero, zero, zero, slash, zero,
being represented with the default keyword.
The ARP table is visible with the ip neigh command.
Ping is pretty universal, ping 192.168.88.1.
Traceroute is simply traceroute 8.8.8.8.
All fairly straightforward.
MacOS uses very similar commands to other Linux distros.
Start by getting to the terminal.
Finder, Applications, Utilities, Terminal.app
The IP address info can be found with ifconfig,
which is a very popular Linux command.
Routing can be seen with the netstat -nr command,
the ARP table is available via arp -a,
ping is the standard ping and the IP address,
and traceroute is as simple as traceroute
and the IP address I'd like to trace to.
I've performed these operations so many times,
I've woken up telling a user to ping something for me.
These commands will be your greatest asset
when troubleshooting any network-related issue.