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37 votes
Accepted

IPv6 subnetting a /64 - what will break, and how to work around it?

If the ISP won't give you more than a /64, then that ISP sucks. If it is any relief I can tell you that I have to deal with ISPs that suck even more than that. Around here it is perfectly normal to ...
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37 votes
Accepted

What is the difference between 0.0.0.0/0 and 0.0.0.0/1?

The 0.0.0.0/0 matches every IP address, whereas 0.0.0.0/1 only matches half of them (0.0.0.0-127.255.255.255) and requires 128.0.0.0/1 as its pair to match the rest (128.0.0.0-255.255.255.255). In ...
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29 votes
Accepted

Does a Gateway of a Subnet need to be a "real" Computer?

Default route (aka gateway address) has to be owned by something that is capable of forwarding packets to the rest of the internet, and which is willing to do so. It doesn't have to be the "principal"...
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28 votes

Connecting to a remote server through a VPN when the local network subnet address conflicts with a remote network

If you need a temporary dirty workaround to a single or a handful known server ips, the simplest solution should be the static client side routing option. In my case I added my desired destination ...
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25 votes
Accepted

Is it possible to SSH access a server with a misconfigured subnet?

You need to be able to log in on another host on the same network segment. Some of the ways to get access to the misconfigured host requires root on the intermediate host, but there also is one easy ...
  • 30k
20 votes
Accepted

How can we explain CIDR notation with /24 and /32 to a manager?

An ip address consists of four numbers separated with a dot between each. You read it from left to right, and each dot adds a level of detail. This is very similar to a real address, think about: ...
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16 votes
Accepted

AWS: NAT Gateway in public subnet. Why?

A NAT Gateway (as well as a NAT Instance, for what it's worth) must be in a public subnet because only devices on public subnets can actually use a public IP address. That's what makes a public ...
15 votes

Does a Gateway of a Subnet need to be a "real" Computer?

The IP address of the gateway can be any valid host IP address in the subnet, i.e. not the network address itself nor the broadcast address. This IP address does not need to belong to a single ...
  • 5,587
14 votes
Accepted

Blocking or rate-limiting IPv6: what size prefix

The best algorithm is to start blocking separate addresses. Then when multiple addresses are blocked in the same /64 you block the whole /64. Repeat that for bigger aggregates. Prefixes are usually ...
13 votes
Accepted

When to use routable vs non-routable IP addresses

Those ranges you list are described in this RFC 1918 - Address Allocation for Private Internets The title gives away their use case: "... for Private Intranets". All IP Address ranges are ...
  • 14.3k
13 votes

Export my whole AWS architecture into file?

You can use CloudFormer to export your existing AWS infrastructure into CloudFormation template. Alternatively you can use terraforming ruby gem to export your infrastructure as Terraform files.
  • 8,304
13 votes

How to pick AWS CIDR within the CIDR ranges of VPC?

The best start is to refer to VPC and Subnet Basics which explains the VPC addressing and sizing reqirements. Not sure if you are familiar with CIDR addressing? Essentially the bigger the number after ...
  • 24.2k
11 votes

IPv6 subnetting a /64 - what will break, and how to work around it?

Yes, pressuring your ISP to not suck is the preferred option. RIR allocation policies assume that the ISP is giving each customer a /48; there is absolutely zero reason for the ISP to not do that. ...
  • 95.5k
11 votes

When to use routable vs non-routable IP addresses

I guess I'm going to be the contrarian on this one. IMO, too many people make too much of a fuss trying to correct people when they talk about class-full ip addressing. The RFC 1918 reserved addresses ...
  • 109k
11 votes
Accepted

I am able to ping a device on a different subnet without passing through a L3 device. How?

Route Print on my local machine shows: 192.168.10.99 255.255.255.255 On-link 10.168.10.154 26 That is why, the On-link routes are accessible locally without any gateway, many times they are ...
  • 253
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 ...
9 votes

Is it possible to SSH access a server with a misconfigured subnet?

kasperd posted a great answer detailed enough for me to learn how I can recover from situation in the question. This answer is the exact step-by-step of how I did it. SSH to server on same physical ...
9 votes

Does a Gateway of a Subnet need to be a "real" Computer?

"Logical entity" in your usage is a tiny bit confusing. But I'll try to answer it best I can. From my experience, a gateway in an IP configuration is usually a physical device. While it doesn't have ...
9 votes

What is the difference between 0.0.0.0/0 and 0.0.0.0/1?

What is the difference between 0.0.0.0/0 and 0.0.0.0/1 0.0.0.0/0 matches all IPv4 addresses. 0.0.0.0/1 matches the IP range from 0.0.0.0 to 127.255.255.255 IP routing uses a "longest prefix ...
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8 votes
Accepted

Is it possible to split an active subnet in aws?

It is not possible to change the CIDR block for a VPC. So if you have created the VPC with 192.168.0.1/24, then that's as big as the VPC can get. You'll have to re-create the VPC to get more space. ...
  • 9,854
8 votes
Accepted

What's the difference between a subnet and a netblock?

I'm not sure that there is an official definition of "netblock", but in common usage, I've most frequently heard it used in reference to an IP address allocation assigned by an RIR to another ...
  • 109k
8 votes

Linux : Ping all the host in the subnet

You can always try to ping to the broadcast address of your subnet. It depends on your local network setup, but you can find it out with $ ifconfig wlp4s0 | grep Bcast inet addr:192.168.199.47 ...
8 votes

How does VLAN subnetting work on IPv6?

VLANs are layer 2 constructs, not layer 3. You pass frames inside a VLAN regardless of layer 3 protocol by switching. For crossing VLAN boundaries, a router is required. Just like with IPv4, if you ...
  • 9,152
8 votes

How can we explain CIDR notation with /24 and /32 to a manager?

You don't need even that detail. Simply treat them as magic values: Short answer: An IP address has four blocks of digits. There is a standard notation called CIDR where if you put /24 at the end it ...
  • 767
7 votes

multiple physical interfaces with IPs on the same subnet

You more likely want to create a bridge with the 8/9 interfaces and then assign an IP address to that bridge (bridge-utils packet, command 'brctl add'). This way the bridge will act like as a switch ...
  • 71
7 votes

Am I meant to allocate a /64 to every host?

IPv6 usually uses a /64 per subnet. Routers send a Router Advertisement or RA to the LAN. This RA contains the basic settings for the LAN like which prefixes (often a single /64) to use, whether the ...
7 votes
Accepted

Ping works but TCP doesn't in a bit of an unusual topology

Your network is working as expected. Lets draw it: This is a simplification I'm sure, but has enough information here to show the problem and the various fixes (and their drawbacks) Assumptions ...
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7 votes

What does the netmask in an IP address actually tell you?

how does the netmask let us distinguish two IP addresses? It doesn't. I lets us or a device distinguish the scope of the network to which it is attached. I don't see how these are two different ...
  • 1,237
6 votes

How does IPv4 Subnetting Work?

While the above is correct (sorry, TL;DR), calculating subnets still causes many network administrators a lot of grief. There actually is a very easy way to do subnet calculation, you can do most of ...
6 votes
Accepted

Is there any harm to use the broadcast IP for a host?

It can introduce performance problems since packets to the broadcast IP address are broadcast to all hosts on the segment. This additional traffic which is intended for only one host but send to every ...
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