All Questions(server)

This is a Canonical Question about IPv6 Subnetting.

Related:

• How does IPv4 Subnetting Work?

I know a lot about IPv4 Subnetting, and as I prepare to (deploy|work on) an IPv6 network I need to know how much of this knowledge is transferable and what I still need to learn. IPv6 seems at first glance to be much more complex than IPv4. So I would like to know:

• IPv6 is 128 bits, so why is /64 the smallest recommended subnet for hosts? Related to this:
  • Why is it recommended to use /127 for point to point links between routers, and why was it recommended against in the past? Should I change existing router links to use /127?
  • Why would virtual machines be provisioned with less than a /64 worth of addresses?
  • Are there other situations in which I would use a subnet smaller than /64?
• Can I map directly from IPv4 subnets to IPv6 subnets? For instance, does an IPv4 /24 correspond directly to an IPv6 /56 or /120?
• My interfaces have several IPv6 addresses. Must the subnet be the same for all of them?
• Why do I sometimes see a % rather than a / in an IPv6 address and what does it mean?
• Am I wasting too many subnets? Aren't we just going to run out again?
• In what other major ways is IPv6 subnetting different from IPv4 subnetting?

I created an Amazon Free tier Usage Account. I launched two amazon ec2 instances using the online tool. After that one instance was created and running while other was pending which quickly shifted to terminated state.

In description it shows
State Transition Reason: Server.InternalError: Internal error on launch

Is there any where I could restart the terminated instance or remove it from table. It looks very annoying

I've seen this with so many consoles (on Linux, Mac, ...), and with lots of different machines in many different networks. I can never pinpoint the exact reason, why this happens: All you have to do is log in to a machine via SSH. If the connection breaks for some reason (for simplicity, let's say the network cable was pulled), then sometimes the console just hangs forever - at other times, it just exits fine to the parent shell.

It's so annoying when this happens (e.g. you lose the command history.) Is there maybe a secret keyboard shortcut which can force an exit (Ctrl-C or Ctrl-D don't work)? And what's the reason for this random "bug" across all the implementations anyway?

I'm running a Rails stack on Ubuntu.

When I call ps -AF, I get a descriptive process name set by the Apache module like

00:00:43 Rails: /var/www...

which is really helpful in diagnosing load issues.

But when I call top, the same process shows up simply as

ruby

Is there any way to get the ps -AF process name in top?

I'd like to start a collection of good, free cheat sheet resources for system administrators. Please add your favorite ones. From the Wikipedia "cheat sheet" article:

In more general usage, a "cheat sheet" is any short (one or two page) reference to terms, commands, or symbols where the user is expected to understand the use of such terms etc but not necessarily to have memorized all of them.