Remote surveillance and Networking (part 2)

8/14/2019 Remote surveillance and Networking (part 2)

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Remote surveillance and Networking (part two)How to make your DVMS accessible: port forwarding

In our July article we discussed redirection of domain name services. We saw that

the IP public address allows communication with the router, and that in turn therouter must be able to interpret this request by forwarding it correctly to the DVMS

(port forwarding). As you now might have guessed, this will be the theme of thesecond part of our series on remote surveillance and networking.

Our objective is simple: to open a communication channel between the client (from

your house as an example) and the server1

(DVMS installed at the surveillance site).To better understand the principle of port forwarding and its numerous components,

lets use the hotel as a metaphor. As an example, when you want to phone a hotel

guest, you first phone the hotel reception (the IP public address managed by the

routers DHCP2 server). You then ask to be put in contact with the hotel guest

(DVMSs fixed address). Reception then transfers you to the guests room (portforwarding). Then the guest picks up the phone and starts talking (servers positive

answer and data transfer).

Now lets apply this metaphor to the port forwarding concept, i.e. in order to make

the communication between the client and the server possible, we must:

1. Assign to the DVMS (server) a fixed IP address3 so the clients requestswill be properly forwarded to it (ex. to assign a room to a visitor).

2. Create exceptions on Windows firewall so that the DVMSs servicesbecome accessible. (ex.to allow the guest to make long distance calls).

3. Redirect one or several ports that are necessary for the client and theserver to communicate (ex. to allow the reception to redirect the calls to

this room).

1. Assignment of a fixed IP addressTo be able to assign a fixed IP address to the DVMS, first we must know where it islocated. As in our telephone metaphor, this would be like saying that Mr. Desjardins

is situated at (514) 940 4346 (routers external and public4 IP address) extension 25

(DVMSs internal and private IP address). To obtain the routers private IP address(also named default gateway), we can type in, from the command window

(Windows, Start, Run) these three letters: CMD followed by Enter. Then we type in

IPCONFIG and then Enter to see the results. You will get 3 types of information: the

1We call the DVMS a server because it serves something namely a Web service. It transfers data from

the cameras and the system.

2The DHCP server (Dynamic Host Configuration Protocol server) is a software component integrated to a

router or to a server. This service works like a traffic cop: it manages IP addresses allowing the redirectionof the requests inputted in the internal network (intranet) and the requests outputted to the external or

public network (extranet or Internet).

3Conversely to the dynamic address that can change in the event of a power outage the fixed address is

called static therefore never changing.

4A public IP address may be obtained by going to the site www.whatismyip.com that will automatically

post it.


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IP address (computer), the subnet mask5 and the default gateway (the routers

internal address). Take note of them. From there, there are two methods that youcan use to give a fixed IP address to your computer:

1. By configuring the routers DHCP server (this point will not be discussedin this article; this method is more complex but has nevertheless the

advantage of better documenting the IP addresses assignments whenusing a local network).

2. By modifying the computer settings for the IP addresses by usingWindows directly. To do this go to: Configuration, then NetworkConnection, right click on Local Network Connection, then click on

Properties. Then double click on Internet Protocol (TCP/IP). Select use

the following IP address then enter the info previously collected. Thenreplace the last number of the IP address with a number found outside

the range6 of IP addresses managed by the DHCP server. And there you

are, you have just assigned an IP address to your server (DVMS). We willnow know exactly where to find you...

2. How to create exceptions to the firewall

When a clients site try to communicate with a server (DVMS) on a specific port,

Windows will automatically consider the request as unauthorized and will post a

message saying that the firewall has blocked this applications service. You must

authorize the execution of these services when requested by the client. To authorizethese services you must create specific exceptions on Windows firewall. Go to

Control panel, then Windows firewall, and click on add port. According to your

DVMS add one or several ports required by your software application. If you belongto a network comprised of several DVMS it would be advisable to use the add a

program option to manage the exceptions. This method will avoid freeing extra ports

to differentiate each DVMS.

3. Port forwarding

When a clients software connects to a server (DVMS), it is in fact a computer on the

Internet sending data to a router (external IP address). When the router receivesthis data, it must know to which computer to forward it. (DVMSs internal IP

address). The port forwarding therefore consists of setting the path needed to

transport this data so that it will be directly forwarded to the DVMS. At the receivingend, the DVMS must be in receiving mode7 that means that one or more

communication ports must be open, waiting to receive the requests. To ensure a

greater connectivity stability with the router it would be advisable to configure theDHCP in static mode. As an example, in a power outage, the IP address would

remain unchanged; this would avoid having to reconfigure the router with a new

dynamic address. In other regards, our router must be able to open some

5A subnet mask shows the number of bits that distinguish the hosts (this in turn indicates also the

number of possible hosts in this subnet).

6To be aware of the range of addresses managed by the DHCP server, you must access the routers

management console using a Web browser. We suggest that you consult the instruction manual provided

with the router.

7Again, with the telephone metaphor, when the telephone rings at your home, if you are absent, you will

not be able to answer, which is the same as if your communication ports were closed.


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communication ports so that direct communication with it is possible. This is similar

to asking Bell to program a different ring to the residential line, the equivalent ofcreating a dedicated line for your beloved teenagers

Therefore, to summarize, there are two steps to follow: first, put the DHCP server instatic mode and then open one or several communication ports (regardless of the

number of services offered8

). To configure these two elements, you must access therouters management console. Since the vocabulary is different from one

manufacturer to another it is somewhat of a risk to explain this procedure in this

introductory article. As an example, port forwarding on a D-Link model is done fromthe advanced thumbnail, then Virtual Server; comparatively the Linksys router port

forwarding will be accessed from the Application link. To avoid possible confusion in

regards to the model of the router that you use, we invite you to consult this link:

http://www.portforward.com/english/router/port forwarding/routerindex.htm

This site offers short guides, and step-by-step instructions supported by screen

captures specifically designed for the model that you use.

It is rather difficult to resume this type of procedure in an article destined to allreaders. However, if you are interested in an in depth look, we suggest that you

consider registering to one of our technical seminars on this subject. Contact us for

the schedule of our seminars presented in our offices or in your region.

Ren St-PierreDirector, Training, Sphere Video

8 As an example, some DVMS can forward audio as well as video. To ensure a better connection, includingthe efficient transfer of data packets the audio services will be delivered by a secondary port.

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Remote surveillance and Networking (part 2)