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Questions tagged [nat]

For questions about Network Address Translation (NAT), a method of modifying network address information in IPv4 packet headers.

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Why are IPv4 addresses running out?

I understand that we are running out (or ran out already?) of IPv4 addresses, but I don't really understand why that is. Right now, every home has its own IPv4 address (dynamically assigned, but still,...
mnj's user avatar
  • 861
7 votes
2 answers
2k views

NAT Overlapping

I am trying to do some networking lab. Basically, I have a network like this: So, is it possible to configure NAT router in such way, that PC1 and PC2 can access remote HOST and each other. The ...
mcferden's user avatar
  • 173
15 votes
2 answers
5k views

How NAT / PortForwarding / TCP/IP works?

I recently read an article entitled how NAT works. Some things still remain unclear to me. I would be thankful if someone could explain. Below is the part of the article regarding DynamicNAT that is ...
user2449761's user avatar
19 votes
2 answers
9k views

How does the router know how to route the packets to my terminal?

Suppose I have a network of 2 computers, each having a private network address of 192.168.1.101 and 192.106.1.102. For my public ip address, it is 10.2.10.172. When I make a http request to google....
Mc Kevin's user avatar
  • 301
10 votes
2 answers
4k views

What do these three options in a Cisco Router NAT configuration mean?

In a standard Static PAT configuration, where one IP:Port combination is always mapped to another IP:Port combination, there are three possible combinations of inside/outside/source/destination that ...
Eddie's user avatar
  • 15.1k
8 votes
2 answers
2k views

UDP hole punching when NAT port distinguished for every outgoing UDP connection?

I just read an article about the process of "punching holes" with the UDP protocol. I understand how it's done, but I have one last question to clear up things. We have the rendezvous server and ...
Tagaeyan Tagan's user avatar
3 votes
1 answer
1k views

How does a router know to which PC in a network to send a response from a server?

When my PC sends a request to a server, the transmitted IP address is the one of my private network. When the server responds to the request, how does the router know to which PC in the network to ...
Marco Menardi's user avatar
2 votes
3 answers
1k views

Why can't a host B act as a server, and accept TCP connections, when sitting behind a NAT?

Originally, I thought I intuitively understood why B can't accept TCP connections, however when I read about how P2P applications circumvent this exact problem by using an intermediate peer that is ...
z.karl's user avatar
  • 123
13 votes
2 answers
31k views

Open a range of TCP ports in Cisco IOS NAT

ip nat inside source static 192.168.1.10 10.10.10.9 route-map RANGE ! route-map RANGE permit 100 match ip address 102 ! access-list 102 permit tcp host 192.168.1.10 range 3000 3389 any The ...
Luna's user avatar
  • 131
10 votes
2 answers
13k views

why STUN doesn't work with symmetric NAT?

I read this line from wikipedia. "STUN does work with three types of NAT: full cone NAT, restricted cone NAT, and port restricted cone NAT. In the cases of restricted cone or port restricted ...
Kristen's user avatar
  • 235
10 votes
1 answer
4k views

How does NAT share one public IPv4 address across multiple private IPv4 addresses [closed]

I think it's a very classical question, but, thought I've have some background, I don't have enough vocabulary anymore to search and understand correctly in the Web. Let's say I have a home network ...
Nicolas Zozol's user avatar
9 votes
7 answers
19k views

What is the purpose of subnetting, if we have NAT?

As I know, NAT is the system, that divides IP addresses into two categories - public and private. Public represents the local network from the outside and private represents the device from the inside....
New Thing's user avatar
9 votes
5 answers
13k views

Why does NAT translate a source port?

I’m beginning to learn about NAT and I was wondering: Why does NAT translate a source port? Doesn’t a port represent an application that’s requesting a service? So why must it be translated? Also what ...
Jaffa's user avatar
  • 91
8 votes
3 answers
5k views

How does NAT decide which connections are inbound, and which are outbound?

I'm building a P2P application that needs to be able to access nodes behind a NAT. This NAT does not allow inbound connections, therefore the nodes outside the network cannot reach out to the nodes ...
Dean M.'s user avatar
  • 83
7 votes
2 answers
3k views

IPv6 and online privacy and security

I am studying up on IPv6 and part of the talk was about how the machines generate or can have multiple IPv6 addresses for internal use and external use. Does this mean that there is no longer a "...
Jason's user avatar
  • 1,015
6 votes
1 answer
3k views

Cisco 867 forward UDP port range

I need to forward an UDP port range to a specific private IP address in my LAN. In order to forward one port to an internal IP I do: access-list <acc_list_number> permit <port_type> any ...
eldblz's user avatar
  • 206
3 votes
3 answers
7k views

At which layer does router operate?

I have studied from various sources that router is a layer 3 device. It does routing based on Network layer (layer 3) header, but I have a doubt. Network address translation (NAT) is a feature of ...
Raja's user avatar
  • 355
3 votes
2 answers
7k views

How is harpinning (NAT loopback) used in practice?

I have never had to use hairpinning in the past and so far, reading its description on Wikipedia, I don't see many use cases for this technology, apart from some requests from selected users inside ...
syntagma's user avatar
  • 349
2 votes
2 answers
597 views

CGN and its advantages

I have been reading about CGN. Though similar to NAT it is used by service providers to route traffic without the need for the a public IP address. But when the packet leaves the ISP to the public ...
john 's user avatar
  • 1,237
21 votes
3 answers
4k views

CEF operations on L3 switch vs router

I understand that L3 switches use CEF for faster packet forwarding. However, CEF will not work for packets that need to undergo NAT. Does this mean CEF isn't used by edge routers and edge L3 switches? ...
Michael May's user avatar
  • 1,005
14 votes
4 answers
38k views

What layer of the OSI model does NAT work?

Simple question and the answer is layer 3 of course, but can we say that PAT is working on layer 4 also because it deals with port numbers which is a layer 4 function? Or is it only a layer 3 ...
aia87's user avatar
  • 143
10 votes
2 answers
12k views

How `UDP Hole Punching` Works?

Can anyone explain how UDP hole punching works in a simple way? I know how NAT and PAT works, but can't get a clear idea about UDP hole punching.
arif's user avatar
  • 613
9 votes
3 answers
5k views

Cisco NAT hairpinning

I have a Cisco router configured NAT (4 static NAT and dynamic NAT). My problem is that I can't access internal servers using public IP addresses from the internal network. I know what the problem ...
sareeshmnair's user avatar
9 votes
1 answer
2k views

NAT VLANs with same IP Subnets

I have a VMware environment in which VMs are running a simulation suite. The software used has hardcoded IP addresses, about 10-15 VMs, and we are running multiple instances of this software each in ...
umhelp's user avatar
  • 93
9 votes
2 answers
9k views

pfSense multi-wan Bridge, NAT, Load balancing and CARP

Context I currently have : 1 pfSense 2.0.2 router (on a Firebox X-Peak X5000) 2 WAN 1 LAN 3 Servers My interfaces WAN1 68.XX.XXX.98 to 69.XX.XXX.102 WAN2 65.XXX.XXX.58 to 66.XXX.XXX.62 LAN 192.168....
Alexandre Lavoie's user avatar
7 votes
2 answers
2k views

PPPoE with NAT configuration broken

We are trying to replace our existing router with a newly purchased Cisco router: GigabitEthernet0/0: PPPoE GigabitEthernet0/1: 192.168.2.1 The PPPoE is getting established perfectly, and I can ping ...
Nick Cameo's user avatar
7 votes
1 answer
3k views

Problem with VPN tunel from remote location through ISP (PPTP)

I have network situation like this: ISP Router (modem) - device given to us by our ISP - modem & router in one but it is set into transparent mode - ISP say so ;D we have to find it that it is ...
DDos Schwagins's user avatar
6 votes
3 answers
3k views

How is NAT a hack?

Forgive me if this sounds a little amateurish, or if this is the wrong stackexchange, but...most of my entirely life I've had ipv4 behind a NAT router. I've been told that this was a hack; simply ...
leeand00's user avatar
  • 331
4 votes
4 answers
764 views

Is there a valid scenario where a router is used to translate a public IP address?

NAT typically refers to router translating an IP address from a private network to a public IP address so that the node on the private network can access public Internet. Is there a valid scenario ...
Jatin's user avatar
  • 143
4 votes
1 answer
1k views

CISCO Port Forwarding Going to Wrong Machine

We are running a few services in our internal network that we would like to expose to the outside world. At first we were having trouble with email, dns, and www. Not sure what happened but email and ...
Nick Cameo's user avatar
4 votes
1 answer
293 views

How are IPv4 Addresses provided uniquely to domains?

this question has been bothering me for sometime and I've yet to receive a clear answer (it bothers me whenever I go to sleep). I would like to receive expert answers on this matter. Who mainly ...
Rave Alroone's user avatar
4 votes
3 answers
2k views

How does a router a layer 3 device translate port address in PAT, as it is a layer 3 device?

While I was reading about PAT i.e Port Address Translation.The router was configured to translate all the private IP addresses to one single public IP.Since router is a layer-3 device, it can do that ...
PRANAY KASTHALA's user avatar
3 votes
3 answers
5k views

IPv6 NAT with Link-Local Address

It's more than usual to find NAT translating IPv4 private addresses (such as 192.168.0.0/16 or 10.0.0.0/8) to non private addresses. Is it possible that NAT translates even IPv6 link-local addresses (...
the structure's user avatar
3 votes
2 answers
2k views

where to NAT?where to run BGP?

in this diagram : we are peering with 2 ISPs, let's say I have a class c IP space like 200.200.200.0/24 and let's say my ASN is 65000, the thing is where should I run NAT? on which device? I mean on ...
user7741's user avatar
  • 435
2 votes
2 answers
1k views

Private and Public IPs in the same OSPF area

I will have a routed network with both public and private subnets routed with OSPF. There will be two core routers that both have a default route to the internet. My question is how I make sure that ...
Peter's user avatar
  • 406
2 votes
4 answers
215 views

Does a Business Router from an ISP perform actual routing or only Network Address Translation (NAT)?

Let's say that the professional business router is connected to the Internet through the WAN port and has some devices in the office connected through the other ports. The devices in the office ...
yoyo_fun's user avatar
  • 1,541
2 votes
1 answer
3k views

Port-forwarding rule on Cisco ASA not working - "Drop-reason: (acl-drop) Flow is denied by configured rule"

New to Cisco, so I hope this question isn't too noobish. I am trying to setup a port-forwarding rule to allow "any" SSL traffic in from "Outside" to the web server on my LAN. The problem is apparently ...
SamAndrew81's user avatar
2 votes
1 answer
1k views

Is NAT hole punching feasible, when the external source port of NAT changed for different destination?

I have a question about hole punching. A-Client B-Client S-Rendezvous server Both A and B connects to S. S learns the random NAT ports of clients. Then sends the IP/Port info to clients. Clients ...
thecat's user avatar
  • 21
1 vote
1 answer
103 views

How to Mitigate DoS by Bindings/Transport Address Exhaustion in NAT64?

Let's say a NAT64 Router only has 1 IPv4 address for bindings, meaning the NAT64 router has a roughly 65535 IPv4 transport addresses. A malicious attacker can then exhaust the entire IPv4 transport ...
maximillian1's user avatar
1 vote
1 answer
166 views

Unsecured devices and IPv6

I'm currently starting to read up on IPv6, and as soon as I heared that every device can/will have a public accessible adreess i got a little freaked out. There are a lot of horror stories about ...
Florian Schöffl's user avatar
0 votes
3 answers
147 views

Do NAT devices need to have private IP addresses?

If a NAT's job is to create mappings between internal ip/port pairs and external ip/port pairs with one Internet-routable IP address, does a NAT need a private IP address for itself? I think one ...
trainsushi's user avatar