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Questions tagged [cidr]

65 questions
55
votes
6 answers

What is the recommended CIDR when creating VPC on AWS?

I have been creating AWS VPCs and I am wondering if there is a recommended CIDR value when creating VPCs. What are the factors that I must consider when choosing a CIDR and does the CIDR value affect the performance of the network?
Gene Diaz
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48
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6 answers

CIDR for Dummies

I understand what CIDR is, and what it is used for, but I still can't figure out how to calculate it in my head. Can someone give a "for dummies" type explanation with examples?
Bob
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25
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6 answers

Linux Command line tool to work with netmasks / CIDR notation

I'm terrible at working out network subnets in my head. Is there some command line tool for linux (ubuntu packages a plus), that lets me put in 255.255.255.224 and it'll tell me that is a /27?
Amandasaurus
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25
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4 answers

Reverse DNS in a CIDR world

Reverse DNS seems to be strongly tied to class boundaries, what methods exist now that CIDR is the standard to delegate authority for a subnet? If multiple methods exist which one is best? Do you need to handle delegation differently depending on…
Zoredache
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21
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6 answers

IPv4 CIDR Ranges for Everything except RFC1918

I'm looking for a list of CIDR blocks for "The Internet", i.e. everything from 0.0.0.0 to 223.255.255.255, excluding RFC1918 address space of 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 (yes, I know there are lots of little nets in there that are…
Jed Daniels
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16
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5 answers

Are Cisco admins expected to understand classful networks?

While studying for the CCENT exam, my reference materials have made an alarming number of references to class A/B/C networks. Thankfully they just treat Class A/B/C as shorthand for /8, /16, and /24 CIDR subnets, and don't make any mention of an…
YouAreTheHat
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13
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2 answers

Grepping for CIDR ranges

From time to time I want to grep CIDR ranges out of my Apache log files. This is easy for ranges that fall on the natural boundaries (/8, /16 and /24) but not so easy for other ranges such as /17 and /25. Examples: # 192.168.0.0/16: (easy) grep "…
Ladadadada
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7
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2 answers

If I change a router's subnet mask to one that includes the old one, will the hosts with the old configuration still work?

Currently I have a router that is addressed as 10.2.1.1 on a 10.2.1.0/24 network. All of my hosts have default gateway 10.2.1.1 with mask 255.255.255.0. I want to know: if I change my router to 10.2.0.0/16 will the hosts that have subnet mask…
tacos_tacos_tacos
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1 answer

What does 192.168/16 mean?

OK, this isn't really a server question in practice, but I'm pretty sure it has something to do with server management. What is this joke supposed to mean?: http://xkcd.com/742
Kudu
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4
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1 answer

Is it safe to assume that two hosts on the same /24 network belong to the same AS-number?

Assume two IP-addresses in the same /24 network (say 1.2.3.*): 1.2.3.X 1.2.3.Y Is it safe to assume that these two hosts belong to the same AS-number? If not, roughly how often is that the case that they do not belong to the same AS?
knorv
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Why is my AWS instance's private IP outside of the subnet's range?

I'm trying to launch an instance with the private IP address 172.31.32.101 using Terraform in my default VPC (CIDR 172.31.0.0/16), in the eu-west-1a subnet that has the CIDR 172.31.32.0/20. I can usually do this with no problem, but more and more I…
scrossan
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4
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2 answers

Is this IPv4 and subnet combination public or private?

Possible Duplicate: How does IPv4 Subnetting Work? I thought I understood this but I am momentarily confused. Here is an example: Are all IPv4 addresses from 192.168.0.0 to 192.168.255.255 in the private range, regardless of subnet mask / routing…
taz
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4
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0 answers

How to support VRF in Netflow Collector?

I am somehow able to analyze netflow packet using my own netflow collector (build in python). It works for certain ip and CIDR(Classless Inter Domain routing), definitely not overlapping. What is the best way to support VRF(Virtual Routing and…
Zangetsu
4
votes
2 answers

Reverse DNS Delegation from ISP

I get a /29 Delegated to me from Verizon(Fios) and I got them to delegate rDNS to my Name servers, I Notice that with some systems that it works and is able to get my PTR record, but on some it isn't able to get the PTR. Is this just a issue with…
Jacob
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2 answers

What is a correct IP4 CIDR for AWS?

I am trying to create some new subnets and cannot get AWS to accept a CIDR IP4. I receive the message: Must be valid IPv4 CIDR. Nothing I try is accepted. What is a valid IP4 for this purpose, and why? See attached image> I tried…
Leon
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