Closed. This question is off-topic. It is not currently accepting answers.
Want to improve this question? Update the question so it's on-topic for Stack Overflow.
Closed 11 years ago.
Improve this question
I would like read netmask, network and broadcast address from an IP address. Basically I’m confused with netmask, CIDR, network and Broadcat terms, could anyone please help me to understand these terms.
Thanks,
Thomman
There is no intrinsic netmask, network and broadcast address for a given IP address. The three terms, combined with an IP address describe a network.
The (CIDR) netmask gives the number of bits that all IPs in the network share. For example, /15 means the first 15 bits are fixed. Because an IPv4 address has 32 bits, the next 32-15=17 bits are then variable. Since every number in an IPv4 address corresponds to 8 bits, that means the following addresses are in the network 1.2.0.0/15 (binary: 00000001.00000010.0.0/15):
1.2.0.1
1.2.0.2
1.2.0.255
1.2.1.2
1.2.255.255
1.3.1.1 # in binary: 00000001.00000011.0.0, i.e. the first 15 bits match
1.3.255.255
but not 1.4.1.1 (00000001.00000100.1.1) or 2.2.1.1 (00000010.10.1.1), since their first 15 bits differ from 00000001.0000001.
You can also express the netmask of a /x CIDR network in binary form by setting the first x bits. In our case, the first 15:
11111111.11111110.00000000.00000000 # binary
255. 254. 0. 0 # decimal
A network address is then the logical AND of any address in the network and the network mask, you set all the variable bits to zero. You can also think of it as the lowest address in the network. In our case: 1.2.0.0.
A broadcast address is the logical OR with the complement of the netmask. You set all the variable bits to one. You can also think of it as the highest address in the network. In our case: 1.3.255.255.
This terminology is equivalent for IPv6 addresses, although you'll usually specify only address and CIDR netmask. Also, a block between two colons now describes 16, not 8 bit. For example, 0001:0002:abcd::/48 contains 0001:0002:abcd::1 and 0001:0002:abcd:12::, but not 0001:0002:abce. You could express the netmask of this network as ffff:ffff:ffff:0 or even 1111111111111111:1111111111111111:1111111111111111::0, but /48 is shorter and easier to read. This network has 48 fixed and 128-48=80 variable bits.
Related
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
Closed. This question does not meet Stack Overflow guidelines. It is not currently accepting answers.
This question does not appear to be about programming within the scope defined in the help center.
Closed 6 years ago.
Improve this question
I know this topic has a lot information all over but I can't find an answer to a simple question.
I am willing to have a subnet for each availability zone in my region (3 zones). My VPC CIDR is 10.0.0.0/19 and I want each subnet to have same amount of IPs. My question is what is the CIDR Block I should assign for each subnet?
10.0.0.0/19 has 8,192 IP addresses, from 10.0.0.0 through 10.0.31.255
When dividing up a supernet into subnets of equal size, you can only divide by powers of two -- 2, 4, 8, 16, etc., so this block can't be divided into 3 blocks of equal size, but it can be divided into 4.
10.0.0.0/21 has 2,048 addresses
10.0.8.0/21 has 2,048 addresses
10.0.16.0/21 has 2,048 addresses
10.0.24.0/21 has 2,048 addresses
Since you only three of these, you could simply reserve one of them for use in a 4th availability zone if you are given access to one (some accounts do have access to more than 3 availability zones in at least one region) or for other purposes.
However, even though you may not realize it yet, you probably need at least two subnets in each availability zone in each VPC. Typically, your instances go on private subnets, but NAT Gateways or Instances and Elastic Load Balancers need to be in public subnets. See Why do we need private subnets in VPC? for more detail on how this works.
So, you probably need at least 6 blocks. Again, you can't make 6 even-sized blocks, but you can make 8, and stash the two leftovers away.
10.0.0.0/22 has 1,024 addresses
10.0.4.0/22 has 1,024 addresses
10.0.8.0/22 has 1,024 addresses
10.0.12.0/22 has 1,024 addresses
10.0.16.0/22 has 1,024 addresses
10.0.20.0/22 has 1,024 addresses
10.0.24.0/22 has 1,024 addresses
10.0.28.0/22 has 1,024 addresses
Another important factor in VPC is that you do not need to worry about the subnet a machine is on if it is communicating with another machine in the same availability zone. There is no difference in performance within an availability zone whether the two communicating instances are on the same subnets or not... so it may make sense to use even smaller subnets that these, or variable length subnet masks, and segregate your machines for administrative convenience.
Closed. This question is off-topic. It is not currently accepting answers.
Want to improve this question? Update the question so it's on-topic for Stack Overflow.
Closed 9 years ago.
Improve this question
How many ways can 1 IPv4 address be interpreted, considering subnet masks? According to wikipedia, it seems like there are 32 possible subnet masks to 1 IP address, but I don't know.
It's not clear to me if a "network mask" is the same as a "subnet mask"; is there such a thing as applying more than 1 mask to an IP address?
network mask and subnet mask are essentially the same thing although there may be different representations. 255.255.255.0 or /24 are both mask representations. A give IP address can fall under different net masks, but there are some standard guidelines. Class A networks are generally lower numbers in the first octet. 10.0.0.0 is a Class A private space with a mask of 255.0.0.0 What that means is 10.anything is part of that network and should not be routed outside that network. you can do things like superneting which means change the subnet to something like 255.252.0.0 which limits the addresses in the network but is not a class a b or c. class B network is 255.255.0.0 . Class C is 255.255.255.0 . Class C is more commonly seen in home environments with a private address space of 192.168.x.0-255. as far as interpreting an ip address, any address is 1 address. the subnet mask tells you if it is a broadcast address(highest address) or a prefix(lowest address) or a valid address in the address space. it also tells a router if it should route the packet or not.
I have the next table:
Network adress NetMask Next Hop
128.96.166.0 255.255.255.0 1
128.96.166.0 255.255.252.0 2
0.0.0.0 0.0.0.0 3
I want to find the adreeses range, for which the router will send to each Hop. for example, the adress's IP: 128.96.166.0 would send to Hop 1.
I don't understand how the NetMak help me to detrmine it. Is it not always would go to hop 1?
The algorithm to decide is:
If (IP address BitwiseAND NetMask == NetwordAddress) Then
Forward to Next Hop
First, start reading the netmask in binary form. In this form the netmask is ALWAYS some 1s followed by 0s. The netmask tells you the length (in bits) of the relevant prefix (1 means relevant bit, 0 means not relevant bit). For instance
128.96.166.1 and 128.96.166.2 would be sent to Hop1 b/c if you only look at the first 24 bits both addresses have the same 24 first bits, just like the first 24 bits of the network address. However the address 128.96.167.1 would not be sent to Hop1, b/c the 24th bit is different.
But the entry for Hop2 has a different netmask. The netmask for Hop2 says to look only at the first 22 bits. When only looking at the first 22 bits 128.96.167.1 matches the network address for Hop2 and packets addressed to 128.96.167.1 would be sent via Hop2.
Further:
When looking at the first 22 bits the network addresses for Hop1 and Hop2 match 128.96.166.1 and 128.96.166.2. However, (normaly) any component will pick Hop1 for 128.96.166.1 and 128.96.166.2 because that rule matches more bits. This is called "longest prefix match" and concurs with the idea "if there are more matching bits this hop will get the packet closer to the final destination".
A simple ip subnet calculator online here: http://www.subnet-calculator.com/
Hi would someone be able to assist with the following question? The question is from a past paper in preparation for an exam.
Consider a router that interconnects three subnets: Subnet 1, Subnet 2, Subnet 3. Suppose all of the interfaces in each of these subnets are required to have the prefix 223.1.17/24. Also suppose that subnet 1 is required to support up to 125 different hosts, and subnets 2 and 3 are each required to support up to 60 different hosts.
Provide three network addresses (of the form a.b.c.d/x) that define the beginning of the IP address range for each subnet, and explain your reasoning.
I think the answer is the following, but I'm not sure.
Subnet 1: 223.1.17.1/25
Subnet 2: 223.1.17.128/26
Subnet 3: 223.1.17.193/26
Regards.
Not quite, Network addresses are always the first Addresses in a Subnet, so the answers would be:
223.1.17.0/25, beginning of IP Address Range: 223.1.17.1 (until .126)
223.1.17.128/26, beginning of IP Address Range: 223.1.17.129 (until .190)
222.1.17.192/26, beginning of IP Address Range: 223.1.17.193 (until .254)
Other than that, your CIDR-Subnet Length is correct, 1 needs at least 126 Hosts (- BC and NA), which justifies /25 -> 24 bits for Class C, 1 bit for Subnet and 7 bit(=2^7 = 128 - Broadcast - Network Address = 126) for hosts, No. 2 and 3 need at least 62 Hosts (-BC and NA) each.
You can see that by using a IP calculator. There are many on the net that also show you some more details..
My favorite (including IPv6) is at http://netools.ch