1
00:00:00,510 --> 00:00:04,140
So what we're doing in this example is source address translation.

2
00:00:04,320 --> 00:00:10,110
We're not going to do destination address translation as source address.

3
00:00:10,110 --> 00:00:15,360
Translation tends to be the most common implementation of net, and that's what you need to know for

4
00:00:15,360 --> 00:00:22,980
the CNA certification with source address translation, you're typically netting devices on your internal

5
00:00:22,980 --> 00:00:26,370
network that use RFC 1918 addresses.

6
00:00:27,110 --> 00:00:34,430
For those devices to access devices on the Internet, their addresses need to be translated or changed

7
00:00:34,430 --> 00:00:39,590
from an RFC 1918 address to a publicly readable address.

8
00:00:39,950 --> 00:00:49,600
In this example, the router has an IP address of 1.1, 1.1, and we're going to net host 10.1, 1.1

9
00:00:49,610 --> 00:00:52,640
to an IP address of 1.1.1 or two.

10
00:00:52,940 --> 00:00:58,400
So in this example, we not netting the PC's IP address to the Router's IP address.

11
00:00:58,880 --> 00:01:01,430
I'll show you that example in a moment.

12
00:01:01,790 --> 00:01:09,920
In this example, we are netting one internal IP address or private IP address to a dedicated public

13
00:01:09,920 --> 00:01:10,940
IP address.

14
00:01:11,510 --> 00:01:16,550
In this example, we've also got a server on the Internet with an IP address of 2.22222.

15
00:01:16,640 --> 00:01:23,660
So the router will populate the net table as follows inside local addresses ten .1.1.1.

16
00:01:23,900 --> 00:01:27,110
Inside global is 1.1. 1 to 2.

17
00:01:27,140 --> 00:01:32,570
Outside local and outside global are set to 2.2.2.2.

18
00:01:32,870 --> 00:01:35,720
In this example, we're not netting the outside address.

19
00:01:35,720 --> 00:01:39,170
We are only netting the inside address.

20
00:01:39,930 --> 00:01:46,620
If you were sniffing this local area network connection between the PC and the router using Wireshark,

21
00:01:46,620 --> 00:01:48,810
and I'm going to demonstrate that in a moment.

22
00:01:48,960 --> 00:01:54,540
What you would see is that the source address of the packet is set to 10.1.

23
00:01:54,550 --> 00:01:55,590
Point one, point one.

24
00:01:55,770 --> 00:01:58,350
That's the physical IP address of this host.

25
00:01:58,590 --> 00:02:02,940
The destination address for traffic is 2.2 to 2.2.

26
00:02:02,970 --> 00:02:09,240
In other words, the PC has initiated a session to the server and if you capture traffic on the local

27
00:02:09,240 --> 00:02:16,230
network source addresses the PC's actual IP address and destination address is the server's IP address.

28
00:02:16,560 --> 00:02:23,220
However, when the traffic hits the rudder, the router will look in the net table because it's receiving

29
00:02:23,250 --> 00:02:30,240
traffic on an inside interface which you configure and the traffic is destined to an outside interface.

30
00:02:30,240 --> 00:02:33,840
So you configure the router with the inside and outside interfaces.

31
00:02:33,840 --> 00:02:39,690
And because of that configuration, the router looks in the net table to see if addresses should be

32
00:02:39,690 --> 00:02:40,440
netted.

33
00:02:40,560 --> 00:02:44,790
In this case, the source IP address matches an entry in the table.

34
00:02:44,970 --> 00:02:48,270
So the source IP address is going to be changed to one point.

35
00:02:48,270 --> 00:02:52,140
1.1 to the destination address is not going to be changed.

36
00:02:52,140 --> 00:02:55,590
It's going to remain as 2.2.22.2.

37
00:02:56,010 --> 00:03:00,360
The router will change the source address and then forward the traffic to the internet.

38
00:03:00,720 --> 00:03:06,870
The source address matches the century of 10.1, point one, point one and the destination address matches

39
00:03:06,870 --> 00:03:09,890
this entry to to 2.222.

40
00:03:09,900 --> 00:03:18,420
So the router is going to change the source IP address to 1.1. 1 to 2 as per the inside global entry

41
00:03:18,420 --> 00:03:19,980
in the net table.

42
00:03:20,190 --> 00:03:26,100
In this example, the outside, local and outside global addresses are the same, so the router doesn't

43
00:03:26,100 --> 00:03:29,160
change the destination address in the packet.

44
00:03:29,220 --> 00:03:35,430
If you sniffed the traffic using Wireshark on this link from the router to the internet, you would

45
00:03:35,430 --> 00:03:44,340
see that the source address is set now to 1.1. 1 to 2 rather than 10.1 1.1, which it was set to on

46
00:03:44,340 --> 00:03:45,840
the inside network.

47
00:03:46,050 --> 00:03:52,380
When the traffic arrives at the server and the server receives the traffic, the server believes that

48
00:03:52,380 --> 00:03:57,240
it's talking with a host with an IP address of 1.1.122.

49
00:03:57,600 --> 00:04:02,730
The server never sees that the actual source IP address was ten .1.1.1.

50
00:04:02,820 --> 00:04:07,710
When the server replies, the source and destination addresses are swapped around.

51
00:04:07,710 --> 00:04:15,870
So the source address is now 2.2 to 2.2 and the destination address is now 1.1.122.

52
00:04:16,140 --> 00:04:21,930
The packet is now routed across the internet and arrives at the net router.

53
00:04:22,050 --> 00:04:28,230
When the router receives the traffic, it sees that the traffic is destined to an IP address of 1.1.1

54
00:04:28,260 --> 00:04:29,010
or two.

55
00:04:29,250 --> 00:04:34,020
The router will check in its net table whether it has a matching entry.

56
00:04:34,020 --> 00:04:35,460
In this case it does.

57
00:04:35,460 --> 00:04:41,310
The inside global address is one point, 1.122, which matches the destination address.

58
00:04:41,610 --> 00:04:50,790
So what the router will do is it will know that the IP address 1.1 and 1.2 and change it to ten .1.1.1

59
00:04:50,790 --> 00:04:54,090
before forwarding the traffic onto the local network.

60
00:04:54,360 --> 00:05:00,870
The source address of quadruple t is not changed because those entries are the same in the net table

61
00:05:00,870 --> 00:05:03,660
for outside local and outside global.

62
00:05:03,840 --> 00:05:10,440
So once again, if you sniffed the traffic between the router and the PC, you would see that the destination

63
00:05:10,440 --> 00:05:11,430
address is 10.1.

64
00:05:11,550 --> 00:05:14,640
1.1 source address is two, two, two, two, two, two, two.

65
00:05:14,640 --> 00:05:21,060
And that's because the router has changed the destination IP address based on the inside local entry.

66
00:05:21,180 --> 00:05:23,910
Now that's essentially how Nat works.

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00:05:23,910 --> 00:05:28,500
This is a simple 1 to 1 mapping with basic net.