I am trying to understand the bits which are structured in an IPv4 or CIDR. Reading through RFC documentation I understood that the IP addresses began as 4 octets of made up of 32-bits. I am confused at what a network mask of 255.255.0.0 means?
Even more over if we look at an IP associated with cnn.com
151.101.65.67
Are the individual bits of the IP listed above
01101001.01000001.00110101.00110111
?
Or am I not thinking about this correctly. The network mask has 255.255.0.0 as an example but how would I look at the actuals bits represented in the octets?
The IPv4 address 151.101.65.67 corresponds to the number that can be represented as hexadecimal 0x97654143 or decimal 2539995459. In binary it would be 0b10010111011001010100000101000011. It is shown as 151.101.65.67 for the sake of human readers (decimal 151 = hexadecimal 97 = binary 10010111, etc., notice that you can simply concatenate the hexadecimal or binary representations to form the full number, but not the decimal!)
A network mask determines which part of an IP address corresponds to the network and which part is a host. It "shows" the network part with a binary '1's and "hides" the client part with binary '0's.
The network mask is 'anded' to an IP address and results in the network address. In your example:
IP address (10010111011001010100000101000011) & mask (11111111111111110000000000000000)
is = 10010111011001010000000000000000, or 151.101.0.0
The network mask is usually only relevant locally. Consider a client with IP address 151.101.66.45 and network mask 255.255.255.0 that wants to view a page on your example IP.
The first decision the client needs to make is: can it connect to the address directly (i. e. is it in the same local network?) or does it need to send the request to the router (gateway). By "&"ing its own IP with its own newtwork mask (result = 151.101.66.0) and comparing it with the remote address & its own network mask (result = 151.101.65.0) it determines that both values are not equal, so the request goes to the router for further communication.
I am trying to learn how to calculate IP addresses from CIDR block.
For example, 10.88.135.144/28 or
10.88.135.10010000/28
From what I understand, that means first 28 bits are associated with network address while the rest 4 bits are host addresses. That would result in following IP range:
10.88.135.10010000 - 10.88.135.10011111
The first IP should be 10.88.135.144 and last IP address should be 10.88.135.159
But according to cidr.xyz. The first IP should be 10.88.135.145 and the last one should be 10.88.135.158.
I really can't figure out why. Can anyone explain the reason for me? Thanks!
Generally, the first IP is the network identifier and cannot be assigned to any device.This is used by router or switch on the network.
The last one is the broadcasting IP and cannot be assigned to any device as this IP is used by router or switch on the network to broadcast information.
https://www.quora.com/In-IP-addresses-what-is-meant-by-network-ID-and-host-ID
https://supportforums.cisco.com/t5/wan-routing-and-switching/what-is-broadcast-address/td-p/2494445#messageBodySimpleDisplay_1
Consider the following classless address block:
154.78.177.3/27
List the addresses from this block that would be used as:
a) the network address,
b) the direct broadcast address, and
c) the range available for hosts to use
Show the steps you took to arrive at your answers.
154.78.177.3/27 means that this ip address is having network address to be 27 bit long ( most significant bit). i.e if we consider the ip address to be 32 bit long. So, in 154.78.177.00000011 ( last 3 number is represented as binary for simplicity) upto 154.78.177.000 (8+8+8+3 = 27) is network address and remaining 5 bit (00000) is for host ip addresses.
Note-
Network address is the first ip address of the total ip's of host.
Direct broadcast address is the last ip address of total ip's of host.
So final conclusion:
1. Network address is 154.78.177.0. This is calculated by setting all 5 bits of host id to 0.
2. Direct broadcast address is 154.78.177.31. This is calculated by setting all 5 bits of host to 1.
3. Range available for host is from 1 to 30 in the last octect. I.e from 154.78.177.1 to 154.78.177.30 ( As first and last ip is reserved for network address and direct broadcast address respectively)
Can someone explain exactly how CIDR blocks work and how it translates into 0.0.0.0/32 for example? Please use laymen’s terms or perhaps even an analogy to something not network related. Can’t seems to find an explanation that clicks with me. Thanks!!
Classless Inter-Domain Routing (CIDR) blocks are for specifying a range to IP addresses in format of IPv4 or IPv6. For the sake of simplicity I will explain rest of this in format of IPv4 however it is applicable to IPv6.
General format for CIDR Blocks: x.y.z.t/p
x, y, z and t are numbers from 0 to 255. Basically, each represents an 8 bit binary number. That's why it is range is up to 255. Combination of this numbers becomes an IPv4 IP address that must be unique to be able to identify a specific instance.
In case of AWS, p is a number from 16 to 28. It represents the number of bits that are inherited from given IP address. For example: 10.0.0.0/16 represents an IP address in following format: 10.0.x.y where x and y are any number from 0 to 255. So, actually it represents a range of IP addresses, starting from 10.0.0.0 to 10.0.255.255.
However for each CIDR block, AWS prohibits 5 possible IP addresses. Those are the first 4 available addresses and the last available address. In this case:
10.0.0.0: Network address
10.0.0.1: Reserved for VPC router
10.0.0.2: DNS server
10.0.0.3: Reserved for future use
10.0.255.255: Network broadcast
See here for official doc.
Actually this is one of the main reasons why AWS permits numeric value of p up to /28. Because for p=30, there will be 4 available values however AWS needs 5 IP address to use. In my opinion for p=29, they might found it inefficient to occupy 5 addresses to provide 3 possible IP address.
Number of possible IP addresses can be calculated by using this formula:
NumberOfPossibleIPs = 2^(32-p) - 5
Classless Inter-Domain Routing (CIDR) block basically is a method for allocating IP addresses and IP routing. When you create a network or route table, you need to specify what range are you working in. "0.0.0.0" means that it will match to any IP address. Some IP addresses are specific, like 10.0.0.0, which will match to any IP address beginning with 10. With any IP address range, you can be more specific by using a suffix(something like /32 from your example). These allow the notation to specify number of bits to be used from Prefix(actual IP-range like 10.0.0.0). It represents the bit length of the subnet mask, as indicated above. The subnet mask is like masking when painting. You place a mask over what you DO NOT want to paint on.
For example, 10.10.0.0/16 will have 256 * 256 IP address in its range.
NOTE: Some of the IP address in a range are reserved for various purposes. According to AWS VPC documentation, following are the reserved IP addresses.
10.0.0.0: Network address.
10.0.0.1: Reserved by AWS for the VPC router.
10.0.0.2: Reserved by AWS. The IP address of the DNS server is always the base of the VPC network range plus two; however, we also reserve the base of each subnet range plus two. For VPCs with multiple CIDR blocks, the IP address of the DNS server is located in the primary CIDR. For more information, see Amazon DNS Server.
10.0.0.3: Reserved by AWS for future use.
10.0.0.255: Network broadcast address. We do not support broadcast in a VPC, therefore we reserve this address.
Hope this helps!
All of the above answers are great, but are missing something pretty important for the people who don't understand addressing.
IP addresses are literally just a string of binary, broken up into 4 "octets". Each octet is a 2^8 block; 00000000. So to a machine, an IP address looks like this (with (.) added for human-ness):
00000000(.)00000000(.)00000000(.)00000000
When we're talking about the "mask" on the IP address, it means "the bits that don't change". The /8 or /255.0.0.0 on the end of the block signifies the number of bits that are not allowed to be used by this network.
So, lets say we have a CIDR block of 10.0.0.0/8 - this can also be written in the format 10.0.0.0/255.0.0.0, and you may in fact see this for of notation in older versions of linux. You will also note that 255 is the decimal representation of the binary string 11111111 - 8 binary "ones". So what the machine sees is the following:
Net: 00001010(.)00000000(.)00000000(.)00000000
Mask: 11111111(.)00000000(.)00000000(.)00000000
The part of the mask with 0's is usable address space within the network.
So the following example addresses are valid in this network, because on the 0 parts of the masked range are changing:
00001010(.)00000001(.)00110000(.)00111000
00001010(.)00110001(.)00110100(.)00111001
When we say "cidr block" we simply mean "the human-readable shorthand way of expressing binary strings understood by a machine". In the above example, the first octet can be expressed as 10, and the latter octets 0. And the Mask can be expressed as 255 and the latter octets of 0, or; because the mask is always a sequence of 1's, then a sequence of 0's, the length of the 1's, i.e. 8
And as such, we get a cidr of 10.0.0.0/255.0.0.0, or 10.0.0.0/8
A few more examples:
-- 172.1.1.0/24
net: 10101100.00000001.00000001.00000000
mask: 11111111.11111111.11111111.00000000
^ 24 bits for the mask ^ 8 bits of usable space
-- 10.10.10.8/29
net: 00001010.00001010.00001010.00001000
mask: 11111111.11111111.11111111.11111000
^ 29 bits for the mask. ^ 3 bits of usable space
Importantly though, this is only one aspect of networking. Usually a couple of these are reserved for things. See other answers for AWS specific things. In their examples, the "first 4" ip addresses reserved for AWS will be the first 4 usable addresses, which would be
...00 - Network address
...01 - Router
...10 - DNS
...11 - Futureproofing
Currenly I have a c function that takes a ip address and subnetmask and checks if given ipv4 is private/public.
It does by following logic to matchsubnet:
(local ipv4 address && local subnetmask) == (given ipv4 && given subnetmask)
What logic can be written for ipv6? I can write a function to receive an ip and subnet prefix. Can i get local subnet prefix?
I order to determine if two hosts are on the same subnet take both their addresses and prefix lengths (subnet masks).
If the prefix lengths are different then they are not on the same network.
If the prefix lengths are the same then take the length ( very probably 64, so let's assume that) and compare the first 64 bits of both addresses with each other. If they are the same then they are both on the same network.
It is basically the same as you do for IPv4 with making the bits that don't being to the prefix. With 128 addresses it might be easier though to implement it as a loop comparing 4 bits at a time (comparing the hexadecimal notation) or 8 bits at a time (comparing the parsed bytes).