I am learning subnetting, I can subnet networks like 192.168.0.0 /16 /24 and so on.
Today I was given this exercise: subnet 192.168.0.1/16, I've never did a subnet on an address with non-zero host part. How am I supposed to do? I looked on Google but found nothing.
There's an excellent Q&A about subnetting over at NESE that you should read.
192.168.0.1/16 is not a subnet address/prefix but a host address within the 192.168.0.0/16 subnet. That you could subnet, of course.
Related
This is a general question about Subnet Masks.
Preface
Most of our private networks are on 192.168.1.xxx/24
The Subnet mask will be 255.255.255.0.
This is the default configuration of any router you go buy at Best Buy.
The Actual Question
Here's what I'm wondering:
Why wouldn't the subnet mask be 192.168.1.0 instead of 255.255.255.0
Assume your ip address is 192.168.1.14
If you 'logical and' the 192.168.1.0 subnet mask with your ip address, you will get the same results as if you 'logical and' it with a 255.255.255.0 subnet mask.
A subnet mask of 192.168.1.0 would be more descriptive than 255.255.255.0.
This is because, a subnet mask of 255.255.255.0 would apply to thousands more potential subnets, a subnet mask of 192.168.1.0 would apply to fewer. Also, a convention where you use a subnet mask like 192.168.1.0 for an ip address of 192.168.1.14 would be much easier to read.
So, why isn't the subnet mask for common private networks, 192.168.1.0?
Why don't we use more specific subnet masks?
Thanks,
Because the specification in section 3.1 of RFC4632 says the masks must have continuous set of 1 bits.
Of course this doesn't really answer the question in any other way than "because it was decided so." You can still think about it: what benefit would it bring? Netmask 255.255.255.0 shows the subnet size of exactly. What size would mask 192.168.1.0 be? How would you describe net mask 255.255.255.192 when the network starts with 192.168.1.0? Impossible.
Also it's a lot easier to say /24 or /27 when the masks are like this.
So in short: they're unambiguous and show the actual size regardless the network address.
I am now maintaining a piece of code that has a warning not to use the value 0 or 255 in an IP address for hosts.
Can someone help and shed some light on the restriction, if any, on IP address for host? It is mandatory to backup the info from a trusted source.
I found a few sources around the internet but non are official or trusted enough.
The most reliable info I found until now is located at RFC1123 section 2.5 and here at the IANA IPv4 Address Space Registry.
Back in the days of Class A, B, and C addresses, a class C address that ended in .0 referred to the entire subnet, and .255 was the broadcast address for the subnet. So these addresses couldn't be used for individual hosts.
However, address classes became obsolete decades ago, when Classless Inter-Domain Routing (CIDR) was developed to make more efficient use of IP addresses. This is described in RFC 4632. The general idea is that each network has an explicit subnet mask, which need not exactly match octet boundaries in IP addresses. So the assumption that .0 and .255 are the network and broadcast addresses is no longer valid.
I don't know which software / library you're using, but I vaguely remember reading this a few years ago about IP address schemes of days long past. In any way, today there's no such rule.
I'm studying subnetting and I can't seem to find a straight answer to what seems like a simple question. Say I have the network 192.168.1.0/26 or in other words, 192.168.1.0 - 192.168.1.63
I have 64 IP's in total and I know the last usable one is 192.168.1.62 since 192.168.1.63 is the broadcast address. Where is the first usable address and how do the Default Gateway, Loopback address and Network IP figure in? I keep getting conflicting info.
For any subnet, there is a "network address" and a "broadcast address". The former is the subnet prefix followed by all zero bits and the latter is the subnet prefix followed by all one bits.
Thus, an N-bit subnet has 2^N - 2 addresses available. (Where N=32-M in A.B.C.D/M notation.)
The "default gateway" is an address on any local subnet where is sent all traffic not destined for any local subnets. It can be any valid subnet address though it's common for it to be the very first address of the subnet on which in resides. A machine can be connected to many subnets (eth0, eth1, wifi0, etc.) but has only one default gateway.
The "loopback address" is not part of a subnet. It is its own interface and has the address 127.0.0.1/8 so any address starting with 127 will reference the local machine.
Note: I have seen configuration options to make the "broadcast address" match the "network address" but I've never seen this used in practice.
I was studying about IP addresses when I came Across an address of type 127.0.0.1/8. I know that127.0.0.1 is a loop-back address but what did /8 mean in the address.what /8 (in 127.0.0.1/8) specify
the /8 in your ip is the subnet mask.
It allows you to know on which subnetwork you are, find your broadcast and unicast IP.
Take a look at http://en.wikipedia.org/wiki/Subnetwork to have more informations
/8 is a subnet mask written in CIDR notation and which is used by hosts to determine if they are on the same network as some other host and can communicate directly or they need to use a router (default gateway). In your case this is a loopback address, and actually all IP addresses from 127.0.0.0/8 network (from 127.0.0.0 to 127.255.255.255) are reserved loopback addresses as defined in rfc6890: Special-Purpose Address Registries
What are the effects of incorrectly setting the netmask? I have a C++ application that sets the network mask of a device. If the netmask is set incorrectly, tftp doesn't seem to work properly. Why would this happen? What other problems occur when the netmask is not properly set for a device/PC?
While this question is probably more about IP networks than programming it is a challenging subject for many developers.
The netmask delimits the host address (your PC or server) and the network address (the part of the logical network infrastructure in which your system lives). The two parts are used to deliver the data packet to the correct device. The network address is obtained by ANDing the netmask with the IP Address. Consider the following scenario:
IP Address: 10.0.1.1
Netmask: 255.255.0.0
The host address portion of the IP address for our PC is 1.1, so the PC knows that any host addresses starting 10.0. are local to it. Any addresses that then start 10.1, etc, are not 'local' and will need to be forwarded to a router. If you have another device intended to be on the same network that is:
IP Address: 10.0.2.1
Netmask: 255.255.255.0
Here the netmask is wrong for our example setup, this device is now going to see the network address as 10.0.2 and the host address as 1, if it tries to communicate with 10.0.1.1 it will see a network address of 10.0.1! Not local and so will refer it to the default router for forwarding. If the netmask was correctly set (i.e. the same as the first example, assuming that's the correct setting for your network) then the second device would see the first as local, i.e. on the 10.0 network and wouldn't attempt to forward the packet to a router.
Many protocols will happily cope with this but tftp is intended to operate within a single network and so will fail as there's a perception that the target is on a different network.
This may not describe your exact situation but I hope that the example demonstrates the important principle that configuration matters, you can't have an inaccurately configured environment and expect it to work.
The netmask determines which IP adresses are local (non-routed); IP adresses outside that range go through the router. If the netmask is wrong, the program tries to directly access sites where it has to go through the router, or vice versa.
The netmask defines, which part of the IP-address is used as address for the network and which part is used for the workstations.
First Example:
IP1: 192.168.20.4
IP2: 192.168.192.4
NM: 255.255.0.0
Both IPs are in the same net. They can communicate with each other without needing a router. That's because the IP-addresses will result in the same bitmask when you or it with the netmask.
Second Example:
IP1: 192.168.20.4
IP2: 192.168.192.4
NM: 255.255.128.0
Now both IPs are in different networks because when you or the IP-addresses with the Netmask, the resulting bitmask will be different and they wont be able to communicate with each other without a router that routes between the two networks.
You can test this by yourself with ipcalc.
Possible implications of mismatched netmask are explained here. In short:
The host is likely to construct routing table incorrectly.
The host will miss some broadcast packets and not send broadcasts properly.
Mis-function of TFTP is almost for sure caused by the first reason. It affects any other IP protocol in the same way.
Other answers mention only the first problem (which is OK, as the second one is rather marginal). Note that it is not the netmask of the interface itself which determines how the IP packets would be routed - it is the routing subsystem of the host; but the netmask is normally used for constructing the routing table.