174
votes
Accepted
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 ...
- 100k
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 ...
- 490
20
votes
Accepted
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 ...
- 100k
19
votes
Accepted
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 ...
- 15.1k
18
votes
Accepted
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, ...
- 1,039
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 ...
- 67.9k
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 ...
- 2,356
15
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 ...
- 326
14
votes
Accepted
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 ...
- 13.6k
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 ...
- 100k
14
votes
Accepted
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 ...
- 100k
13
votes
Accepted
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: &...
- 478
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 ...
- 100k
12
votes
Accepted
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 ...
- 100k
11
votes
Accepted
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 ...
- 4,487
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 ...
- 67.9k
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 ...
- 19.8k
10
votes
Accepted
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).
...
- 86.4k
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 ...
- 100k
9
votes
Accepted
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 ...
- 100k
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 ...
- 67.9k
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....
- 15.1k
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 ...
- 6,780
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 ...
- 13.6k
8
votes
NAT and UDP problems
UDP poses a problem to NAT as there is no traffic to indicate whether a connection (source/destination port pair) is in use any longer or not. UDP reverse routes have to be aged out generally - in ...
- 86.4k
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 ...
- 8,846
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 (...
- 86.4k
7
votes
How does NAT decide which connections are inbound, and which are outbound?
A TCP connection is considered to be "in force" as long as the software processes on either end keep the socket open.
When the process(es) on one or both ends close the socket (either gracefully or ...
- 2,426
7
votes
Accepted
routing between vlans not working
The problem is that you have not defined the ACL that tells the router which addresses to translate.
You have:
ip nat inside source list nat-list interface GigabitEthernet2 overload
but you do not ...
- 100k
Only top scored, non community-wiki answers of a minimum length are eligible