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174 votes
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Why are IPv4 addresses running out?

The IPv4 Address Shortage According to Vint Cerf (the father of IP), the IPv4 32-bit address size of was chosen arbitrarily. IP was a government/academic collaborative experiment, and the current ...
Ron Maupin's user avatar
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39 votes

Why are IPv4 addresses running out?

Ron Maupin's answer gives a brilliant overview of the IPv4 shortage, but I'd like to address this part of your question: Why can't a city (for example) have just one IP address and all homes in ...
IMSoP's user avatar
  • 490
19 votes
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IPv4 Segment 100.64.0.0/10

The 100.64.0.0/10 address block is not private address space; it is shared address space. This is spelled out in RFC 6598, IANA-Reserved IPv4 Prefix for Shared Address Space (I highlighted the ...
Ron Maupin's user avatar
  • 99.8k
19 votes
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Is there anything stopping me from using Class A addresses on my own network?

As long as you are translating your "15.0.0.0" address space to something unique on the Internet that doesn't overlap, things will "work fine". However, you won't be able to ...
Eddie's user avatar
  • 15k
17 votes

IP addresses from public IP block in my LAN

EDITED I'm assuming you're considering using an IP block that is not registered to you. Otherwise, skip to the last paragraph. Besides being a very poor practice, if you use public addresses on your ...
Ron Trunk's user avatar
  • 67.6k
17 votes
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Why is symmetric NAT called "symmetric"?

tl;dr It comes from STUN and is symmetric because it identifies a 1:1 relationship between NAT mapping and L4 flow. The name symmetric NAT is as far as I know introduced by the original STUN RFC 3489, ...
KillianDS's user avatar
  • 1,029
16 votes

Why are IPv4 addresses running out?

Right now, every home has its own IP address. Why can't a city (for example) have just one IP address and all homes in this city would just be on a private network of that city? Exactly this is ...
Martin Rosenau's user avatar
14 votes
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How `UDP Hole Punching` Works?

Details vary but basically it goes something like. The two peers both open a UDP socket bound to a random local port The two peers both contact a server on the internet. This server responds and ...
Peter Green's user avatar
  • 13.3k
14 votes

Why does NAT translate a source port?

First, I'm assuming you're focused on TCP. UDP has some differences, and I'm not as up-to-speed on that part. I’m beginning to learn about nat and I was wondering why does NAT translate a source ...
Slartibartfast's user avatar
14 votes

Could IPv6 make NAT / port numbers redundant?

IPv6 does not have a NAT standard as IPv4 does (NAT breaks the end-to-end premise of IP, and IPv6 was designed to restore that). There is an experimental RFC for IPv6 NAT, but it is a one-to-one NAT ...
Ron Maupin's user avatar
  • 99.8k
14 votes
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Does NAT violate end to end argument?

It means that IP was designed for each endpoint only to maintain the state of the communications. NAT requires that the NAT device in the middle to maintain a state of the communications. IP was ...
Ron Maupin's user avatar
  • 99.8k
13 votes
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IPv6 and online privacy and security

Let's look at a live example. This being from my Linux workstation. (And for simplicity I actually omitted a couple of addresses.) I'll explain each of the addresses in turn: $ ip a s dev br0 3: br0: &...
Michael Hampton's user avatar
12 votes

Is there anything stopping me from using Class A addresses on my own network?

Any network addresses you use in your own company that are in use or assigned to a different company on the public Internet will be inaccessible to your users trying to reach those addresses on the ...
Ron Maupin's user avatar
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12 votes
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How does NAT Stitching work?

You must remember that a flow using NAT will look like two different flows: a flow pre-NAT, and a flow post-NAT. This is because NAT is changing one or more of the addresses in the packets. This can ...
Ron Maupin's user avatar
  • 99.8k
11 votes
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Ping a private IP address in another network

A PC in a private IP range can't be acccesed from the public internet. Devices in private range connecting to internet use a proxy or router/NAT device that replaces the local source IP for a single ...
jcbermu's user avatar
  • 4,497
11 votes

routing between vlans not working

You don't have NAT enabled (you don't have ip nat outside on Vl 20). Without NAT, the ISP doesn't know how to route back to your 192.9.200.0 network. If you NAT to the VLan 20 address, the ISP ...
Ron Trunk's user avatar
  • 67.6k
11 votes

Do I get any security benefits by NATing a network that's already behind a firewall?

No. I agree with @RonMaupin in the comment: NAT is not a security feature but a workaround for the lack of IPv4 addresses. NAT only add complexity without increasing security, if the network and the ...
JFL's user avatar
  • 19.7k
10 votes
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What is the purpose of subnetting, if we have NAT?

NAT and subnetting solve two different problems. Problem 1: Previously IP addresses were divided into classes A, B and C. A class C address had a default subnet mask of 255.255.255.0 meaning 24 bits ...
Jimmy's user avatar
  • 606
10 votes
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How are IPv4 Addresses provided uniquely to domains?

Who mainly provides IP addresses to domains? As in, who commands, "www.google.com, 103.233.38.93 is yours; www.stackexchange.com I will assign 104.16.115.182 to you; etc." Normally IP addresses are ...
Peter Green's user avatar
  • 13.3k
10 votes
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How does UDP NAT Know When To Remove The Rule

A NAT router doesn't know when to remove a UDP mapping - it guesses. The router simply ages (or times) out the entry when it hasn't been used for a period of time (usually between 5 and 60 minutes). ...
Zac67's user avatar
  • 84.8k
10 votes
Accepted

At which layer does router operate?

Network address translation (NAT) is a feature of Router which is required for routing traffic. That is completely incorrect. NAT is a kludge (a clumsy, inefficient solution) designed to extend the ...
Ron Maupin's user avatar
  • 99.8k
9 votes
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How does NAT decide which connections are inbound, and which are outbound?

TCP is a connection-oriented protocol, and it can only communicate through connections. Before you start programming using TCP, it would be helpful to first understand how TCP works, and you should ...
Ron Maupin's user avatar
  • 99.8k
9 votes

Could IPv6 make NAT / port numbers redundant?

One drawback is that the upper layers would need to be aware of IP addresses, which sort of violates the layering principle. What would then happen if you switched to IPv4? Or something else? How ...
Ron Trunk's user avatar
  • 67.6k
9 votes
Accepted

Why isn't 10.0.0.0/8 used instead of 192.168.0.0/16 for private addresses?

It is merely a necessity constraint and personal preference. If a Network is being built that would only have 10, or 20, or 50, or even 100 hosts, there is no reason not to use a /24 from 192.168.0....
Eddie's user avatar
  • 15k
8 votes

What layer of the OSI model does NAT work?

Nat is a cross-layer process. It involves at least layers 3 (IP) and 4 (TCP, UDP, etc). In some cases it can also involve layer 7 (application). A one to one NAT needs at minimum to modify the IP ...
Peter Green's user avatar
  • 13.3k
8 votes

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

Let me see if I can clear up some misunderstanding for you: NAT is the system that divides IP addresses into two categories - public and private. Public represents the local network from the ...
Ron Trunk's user avatar
  • 67.6k
8 votes

IPv4 Segment 100.64.0.0/10

That block of addresses is reserved for service providers to be able to do NAT in such a way that it doesn't conflict with the normal private address space. If you start using it as private space then ...
Sander Steffann's user avatar
8 votes

IPv4 Segment 100.64.0.0/10

Officially RFC 6598 says Devices MUST be capable of performing address translation when identical Shared Address Space ranges are used on two different interfaces. Most NAT ...
Peter Green's user avatar
  • 13.3k
8 votes
Accepted

NAT VLANs with same IP Subnets

With a bit of VRF-lite and VRF-aware-NAT and the help of the Cat-3850's routing capability, here's some config snippets that should work, or at least get you halfway there - all based on the diagram ...
Marc 'netztier' Luethi's user avatar
8 votes

TCP connections hang during handshake

You state that TCP connections run fine once they get established. That makes a cause on the ISP's network unlikely: ISP routers are usually stateless, so they don't care for layers above the network (...
Zac67's user avatar
  • 84.8k

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