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179 votes
Accepted

How bad is IPv4 address exhaustion really?

It's very bad. Here is a list of examples of what I have first hand experience with consumer ISPs doing to fight the shortage of IPv4 addresses: Repeatedly shuffling around IPv4 blocks between cities ...
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138 votes

How bad is IPv4 address exhaustion really?

Before we started to run out of IPv4 addresses, we didn't (widely) use NAT. Every internet connected computer would have its own globally unique address. When NAT was first introduced, it was to move ...
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91 votes
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What's the maximum number of IPs a DNS A record can have?

Disclaimer: No offense, but this is a really bad idea. I do not recommend that anyone do this in real life. But if you give a bored IT guy a lab, funny things will happen! For this experiment, I ...
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89 votes

Do you need separate IPv4 and IPv6 listen directives in nginx?

If you host multiple vhost domains with a single Nginx instance, you can't use the single combined listen directive listen [::]:80 ipv6only=off; for each of them. Nginx has a weird quirk where you ...
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42 votes
Accepted

What makes a private IP address not routable?

Private IP addresses are routable, albeit they are not publicly routed. Basically, a router will route a private address to private/internal LAN, rather than to the internet. To expand my answer: a ...
38 votes

Should my website have an IPv6 address?

Lack of IPv6 support on your site will hurt some of your users. According to stats published by Google 20-25% of users currently have IPv6. A large fraction of those users will need to go through ...
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34 votes

New service decision: IPv6 or IPv4?

Both of course. IPv4 will stay a long time, and it's way past time to start with IPv6.
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31 votes
Accepted

New service decision: IPv6 or IPv4?

IPv4 and IPv6 are separate protocols that don't talk to each other. You'll have to support both protocols for now. Getting IPv4 addresses is getting more difficult and expensive, but you'll have to ...
30 votes
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Difference between 0.0.0.0/0 and ::/0 in access control lists

0.0.0.0/0 is the IPv4 everything - all possible IPv4 addresses. ::/0 is the IPv6 equivalent of that. You can, for example, allow IPv4 and disallow IPv6 or vice versa. @kasperd mentions: It should ...
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27 votes
Accepted

Is an IPv6-only MTA possible yet?

Short answer: it will work, technically, but you will have lots of undeliverable mail. Long answer: Take your SMTP logs. Sed out all the domain names you send mail to. Check if they have IPv6 DNS and ...
  • 3,497
23 votes

Prefer IPv4 incoming connections over IPv6

IPv6/IPv4 preference is determined by the initiator of a connection, i.e. the web browser. The address selection rules are defined in RFC 6724. While these can be overridden, it is only by the user ...
22 votes

Is an IPv6-only MTA possible yet?

The answer depends on your success criteria. But most likely will be no. If you are running a business where any undelivered mail means a measurable cost. Then the answer is no, IPv6-only is not ...
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22 votes

How bad is IPv4 address exhaustion really?

One big symptom of IPv4 exhaustion I didn't see mentioned in other answers is that some mobile service providers started going IPv6-only several years ago. There's a chance you've been using IPv6 for ...
22 votes

What is the name of an IP prefix with prefix length 31? (i.e. contains only 2 IP addresses)

A /31 is most often used for creating a point to point link between two networks, since it can do so by only using two addresses. Hence the name, 'linknet' You can read this RFC for more information.
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21 votes
Accepted

American Users have wrong IP address geolocation

Maxmind is a good service, though occasionally there can be errors, since we're now in the time period where IPv4 blocks are scarce, and are being traded and resold on a gray market. If you do find an ...
21 votes
Accepted

HTTPD listening in IPv6, according netstat, but reacheable in IPv4

I believe that on Linux, binding to [::] (IPv6) results in receiving both IPv6 and IPv4 traffic (by default). I believe these are referred to as IPv4-mapped IPv6 addresses. netstat simply shows the ...
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21 votes

IPv4 Properties Settings for servers on an isolated network?

Leave the default gateway blank. There is no default gateway, because there's no access to any other subnets.
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18 votes

Domain connection shows as "unauthenticated"

One possible reason for this issue is when the machine account password gets out of sync with the domain controller. This can happen, for example, if the computer account in Active Directory is ...
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18 votes

What's the maximum number of IPs a DNS A record can have?

To answer the question as it was stated ("how many IPs can a single DNS A record hold?") the answer is very simple: a single A record holds exactly one address. There can however be multiple A records ...
18 votes
Accepted

How do I make actual use an IPv4 block that I own?

There are two things you need: First you need an ISP that will act as the sponsoring LIR for you. Their role is just book keeping and maintaining the contractual chain between you and RIPE NCC. Then ...
17 votes
Accepted

How do I get anycast for my servers?

There are two separate aspects about anycast that need to be understood in order to address your particular request. The first part is how anycast addresses are advertised and routed. The second is ...
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15 votes

Prefer IPv4 incoming connections over IPv6

Such preferences can be expressed using SRV records. Unfortunately those are not supported for HTTP. So you are left with a situation where the client alone is making the choice between IPv4 and IPv6. ...
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14 votes

Carrier Grade NAT concepts: IP bans

That is indeed one of the problems with CGN. Sharing a resource means that all suffer the consequences when one abuses the resource. A bank that I consulted for implemented IPv6 on the server side ...
14 votes

How bad is IPv4 address exhaustion really?

The major RIRs ran out of space for normal allocations a while ago. For most providers therefore the only sources of IPv4 addresses are their own stockpiles and the markets. There are scenarios in ...
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13 votes
Accepted

OpenVPN with mixed ipv4 and ipv6 clients

Well. On the server side, specifying "proto" twice doesn't actually do anything - "proto udp6" will make it bind a dual-stack socket to handle v4+v6, overwriting the "proto udp" in the previous line. ...
13 votes
Accepted

IPv4 Properties Settings for servers on an isolated network?

It really doesn't matter what you put as default gateway. All machines are in the same network thus no routing needs to happen so no gateway to another network needs to be contacted. --edit-- And yes, ...
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12 votes
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Can the network still work when DNS & DHCP go offline?

The network will keep working until your DHCP leases expire. After the leases expire devices may switch to RFC 3927 addresses. But those addresses are not predictable so you'd have to rely on MDNS to ...
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10 votes

What's the maximum number of IPs a DNS A record can have?

Each IPv4 address will take up 16 bytes in the reply. Each IPv6 address will take up 28 bytes in the reply. It is strongly recommended that you ensure the reply will fit in 512 bytes. That would ...
  • 30k
10 votes

How does IPv4 Subnetting Work?

I’ll pose and answer a few related questions along the way: Why do you see 255.255.255.0 so often? Why 192.168.0.1? Why 127.0.0.1? Why such weird numbers — 255, 192, 168, 127? 8+8+8+8-bit dotted ...
10 votes

No IPv6 & DNSSEC support on cc-TLD? (practical implications)

If the ccTLD does not have IPv6 addresses for its name servers, an IPv6-only user may not be able to resolve any names under that TLD, even if those names are in IPv6-competent zones. Resolving ...

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