Video Transcript
Quantum Networking (Part 3)
10m 23s
00:07
Welcome back.
00:09
Up till now we discussed a lot of ideas about IP addresses and subnetting,
00:14
but, now let’s look at some specific ways to understand and troubleshoot a problem in the real world.
00:20
Two very useful tools for troubleshooting IP addressing and problems with subnets
00:25
are actually built into the windows environment.
00:28
The PING command can help assess basic network connectivity.
00:32
This can be called executed from the windows command prompt.
00:35
You can PING a specific IP address and confirm there’s a reply from that address.
00:41
Remember those OUIs we discussed before?
00:44
You can look at the address resolution protocol (or ARP) table on your machine
00:49
as well to confirm the correct piece of hardware actually replied.
00:53
If you know the full MAC address or at least the OUI of the manufacturer you expect,
00:59
you can make sure the correct MAC address is associated with the IP address in question.
01:04
This is especially useful in finding duplicate IP addresses on a network.
01:09
If you know a device has a certain IP but the MAC address doesn’t look right,
01:13
then another device most likely has been assigned that IP address as well.
01:18
Let’s take a look at another example.
01:21
Say I’m on a typical Q-SYS project site, where I have a wired connection to the Q-SYS network
01:26
and a wireless connection to the end user’s guest network for email, chat etc.
01:31
Let’s say the Q-SYS core is set up with the IP address 192.168.2.2 with a subnet mask of 255.255.255.240.
01:43
I set my computer’s wired connection manually to 192.168.2.17 with the same netmask as the core.
01:52
As a preliminary network connectivity test,
01:55
I PING the core from the windows command prompt and I get a ‘destination host unreachable’ message.
02:02
Why don’t I get a response?
02:05
If I look carefully at the subnet mask (or just use a subnet calculator),
02:10
I can see that the subnet of the Q-SYS Core has a valid range of 192.168.2.1-14,
02:19
so 17 is out of the allowable range.
02:23
And if I look closely at what the computer is doing,
02:25
I’ll notice that the PING request isn’t even being sent to the Q-SYS network.
02:30
It’s actually being sent to a wireless pathway to the internet,
02:34
as that’s how the operating system handles traffic to other subnets.
02:38
It attempts to route them.
02:40
That's because IP routing is a technology that operates at OSI layer 3.
02:45
It makes use of routers to connect LANs on different subnets,
02:49
or LANs to Wide Area Networks (WANs) or the internet itself.
02:54
This layer 3 routing technology is what allows us to reuse these private address schemes
03:00
all over the world (in our cities if you will) while still allowing access to the internet.
03:05
A router has an internal address, known as the ‘gateway’ address, and an external address.
03:12
This may be a router directly connected to the internet where all addresses are distinct,
03:18
or may be connected to a LAN of routers that are connected to other routers.
03:22
Many packets routed to the internet are routed many times before actually reaching their destinations.
03:28
Routers communicate to the outside world on behalf of all the devices on its LAN side.
03:35
They inspect and route the replies from the outside world back to the appropriate device on the LAN.
03:42
For the devices on the private side of a router to connect to the public side,
03:47
we need to specify a third parameter in the IP configuration – the gateway address,
03:53
which represents the private address of the router itself.
03:57
There are a couple of things we need to be careful of when specifying a gateway address:
04:02
A) it needs to be a valid private router address,
04:05
and B) That address must be on the same subnet as the device itself.
04:11
Let’s look at an example: Let’s say a device has IP address 192.168.1.101 and it's on a class C subnet.
04:22
We specify a gateway of 10.36.121.7.
04:28
That’s problematic, as we thenn would need a router to talk to the specified gateway,
04:33
which IS our router address.
04:36
We need to make sure the gateway address is directly accessible by the device we’re configuring.
04:42
Very often, the gateway is either the first or the last available address on the subnet in question.
04:48
One other note – specifying a gateway is NOT mandatory.
04:53
If a device is on an isolated network and doesn’t need to communicate with devices on another subnet,
04:59
there is NO NEED to define this at all.
05:01
You can actually run into some subtle problems if you pick an address that isn’t a valid router.
05:07
And now that we’ve discussed the basic parts of configuration,
05:10
let’s look a little more closely at exactly what a router does.
05:15
Let’s say I have my lonely little computer who would like to browse the internet.
05:20
The computer uses another technology (that we’ll discuss later)
05:23
to find the website it wants to access is hosted at address 98.139.80.80 on the internet.
05:31
The computer passes the packet bound for that address to the gateway address of the router.
05:37
In the simplest systems, the router takes the packet and replaces the private source IP with its public IP address
05:44
to pass through the network of routers and relays that comprise the cloud.
05:48
The reply from the destination address will be sent to the router’s public IP.
05:53
The router will then pass the reply back to the original requestor on the private side.
05:58
This is often referred to as Network address Translation or NAT.
06:03
It comes in a number of forms, but this is how it works in the general sense.
06:07
Note that there are many schemes for IP routing, but the basic concepts are the same – you’re sending packets from one subnet to another with the router as a go-between.
06:18
Now, we’ve talked about the need for IP addresses on the network,
06:22
but we haven’t yet discussed how those are configured on the devices themselves.
06:26
There are a few different possibilities here.
06:29
In general, there are two models:
06:31
Automatic IP addressing, which means the user doesn’t need to do anything to assign one.
06:36
The most common protocol for this is the Dynamic Host Control Protocol, or DHCP.
06:43
Using this model, a device requests an IP from a machine on the network that serves as a DHCP server.
06:51
The server manages the addresses of each device and makes sure they each get the right configuration,
06:57
including subnet mask, gateway address, etc.
07:00
The BOOTP protocol is an older dynamic addressing protocol that works in a similar fashion.
07:06
It’s most commonly used by older embedded networking devices.
07:10
Most modern DHCP servers will respond to BOOTP requests.
07:15
If a device is configured for an Automatic IP addressing and there’s no DHCP server on the network,
07:21
it will often use the APIPA scheme to assign one itself.
07:26
If you see an IP address starting with 169.254 this is most likely the case.
07:31
These are defined as Class B networks.
07:35
If a device receives no reply from a DHCP server and it supports APIPA,
07:40
it will send out a broadcast that says
07:41
‘Hey, I’m thinking about taking on this address in the APIPA scope, does anyone object?’.
07:46
And if another device doesn’t reply saying it already has that address, it assumes it’s safe to use.
07:53
If it does get an objection, it will repeat with a different address.
07:57
And then finally, Static addressing means the user must know what the IP of a device should be
08:03
and enter it manually along with the subnet, etc. in the device's configuration.
08:09
Let’s have a look at how these configuration options apply directly to a Q-SYS core.
08:14
If we look at a core configuration in core manager, we see the mode,
08:19
whether static or automatic for each Q-SYS network interface.
08:23
Core manager shows us the MAC address of each Q-SYS interface as well.
08:27
The Link Layer Discovery Protocol,
08:31
or LLDP shows us the MAC of the switch port to which the interface is connected.
08:37
For interfaces in ‘Auto’ mode with no DHCP server, we see the APIPA support here.
08:43
Note Q-SYS devices are configured as ‘Auto’ out of the box,
08:47
so it’s very common to see this until other configuration options are used.
08:52
The gateway address will be used to communicate with other subnets.
08:56
Note that only ONE gateway should be configured for the combination of all interfaces.
09:01
We’ll explain why later.
09:03
We have one additional configuration option that should be discussed.
09:07
The Domain Name (DNS) Server or DNS configuration.
09:11
A DNS server is a computer who translates domain names (such as QSC.com)
09:18
to IP addresses so that devices know how to address a packet.
09:22
If set to “Auto”, the DHCP server will provide the DNS server information to the interface in that case.
09:29
If it isn’t assigned automatically or if the interface is in ‘Static’ mode,
09:34
it must be manually enabled and have servers manually configured.
09:38
If manual DNS configuration is needed, consult the network administrator for the correct address. As a troubleshooting measure, you could try a Google DNS address.
09:49
Google’s DNS servers, 8.8.8.8 and 4.4.4.4 are always good bets when manually configuring devices.
09:57
So, when do you need a DNS server?
10:01
Well, hosted SIP providers will typically provide a domain name rather than an IP for registration,
10:07
so that’s one example.
10:08
Others are NTP time servers who sit on the internet to keep devices synchronized.
10:13
Alright let’s take a break here, before we start talking about different types of network traffic.
Downloads and Links
Quantum Networking (Part 3)
10m 23s