If it is the subnet mask that identifies a network and therefore by exclusion, the machine in this network, then only one address can correspond to several machines. For example 164.24.86.86.18/16 does not correspond to the same address as 164.24.86.18/24. So for example when I enter an IP address without its mask in my browser, how can it find the machine without its mask?
The IP address and netmask serve different purposes.
The IP address uniquely identifies a networking endpoint.
The netmask is used by software and hardware between the originating endpoint and the destination endpoint to filter or categorize packets with a given IP.
For example, you may wish to block all traffic originating from Amazon Web Services. You can download the file containing all AWS network ranges, which specify those ranges as netmasks. For example, 18.208.0.0/13. Then you can run iptables to block traffic with IP addresses covered by that mask (for example, 18.208.12.34).
Similar filtering happens inside routers. For example, in a network with multiple physical subnets, you can use netmasks to direct traffic to the appropriate subnet.
Your example 164.24.86.18/24 is, by the way, invalid. It should either be 164.24.86.18/32 (which masks all but a specific IP address) or 164.24.86.0/24 (which masks the /24 network range starting with 164.24.86).
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In a global internet, for two machines on the same subnet to communicate, they need to know each other's physical address. So, the source machine has to map the internet address of the destination into a physical address. Why is this address mapping important? Couldn't the two machines just communicate using their internet addresses?
Because the goal of the Internet is to connect different types of subnets together. It is local decision of each subnet how to organize addressing on this subnet and how to deliver packets on it. Thus layer 3 (that uses IP addresses) delivers packets up to the subnet, and then the subnet (layer 2) decides how to deliver packets within itself.
Mapping, that you are describing is done for Ethernet-type subnets. One can potentially have different types of subnet protocols which do addresses differently. Although, now, almost everyting falls under Ethernet-family.
I recently got to know about DHCP that it dynamically assigns ip addresses to a computer. But what if two DHCP servers across the world assign a same ip address to two different computers. Doesn't it conflicts with the uniqueness property of ip addresses.
For example two DHCP servers assign ip address x to computer1 and computer2 and i wanted to send a mail to computer1 by looking at DNS server it should be send to ip address x. But there is a conflict as two different computers have same ip address x. could someone please clear my doubt.
Let say I want to contact a server whose domain name is example.Com and ip address stored in DNS server is 127.18.1.1 and DHCP changes the ip address for this server to some random value 127.19.1.1 then how is the address resolved since I'm contacting to example.com which has 127.18.1.1
There are a number of ways that this problem is avoided.
Firstly, by keeping networks small, segregating machines into specific functions, geographic areas, etc you reduce the risk of having a large pool of addresses in use.
Secondly, if you must have multiple DHCP servers on a network, then the address space should be split between them. So if you have 192.168.0.0/24, one server would handle 192.168.0.0/25 and another would handle 192.168.127.0/25
Thirdly, most DHCP servers will ping the IP address that they are going to offer before offering it. If the address gets a response it will be marked as Conflicted and won't be offered again.
I'm exploring alternate multiplicities between IP subnets and VLANs, outside the recommended 1-to-1 implementation. My understanding is as follows:
Multiple subnets to a single VLAN (connected via a switch):
Hosts across both subnets would receive layer 2 broadcasts (such as ARP), but would ignore traffic lacking an IP that targets them.
Question: Would I be able to communicate across subnets without a layer 3 device if I could manually insert a destination MAC address in the frame header? My understanding is that the layer 2 switch is oblivious to the differing subnets, and assuming it knows the location of the destination MAC address, would forward the packet in its direction. The destination PC, seeing its IP and MAC addresses, would accept the packet, effectively letting it cross subnets without ever being routed.
A single subnet across multiple VLANs:
Broadcast traffic would be isolated to the individual VLANs. This would break ARP, as a host targeting another machine in the same subnet (but unknowingly in another VLAN) would send out an ARP request that would never be responded to.
This would effectively create separate, identical address pools for each of the VLANs, though I'm not sure how a router would differentiate between the two when interVLAN communication is attempted. I'm a little bit unsure about the pros/cons of this configuration..
Why would we ever want to do this?
Multiple subnets to a single VLAN (connected via a switch):
Hosts across both subnets would receive layer 2 broadcasts (such as ARP), > but would ignore traffic lacking an IP that targets them.
This actually has it's use case in modern DCs. Not in a way you suggest it (w/o a L3 device), but with a VEPA switch.
A single subnet across multiple VLANs:
Broadcast traffic would be isolated to the individual VLANs. This would break ARP, as a host targeting another machine in the same subnet (but unknowingly in another VLAN) would send out an ARP request that would never be responded to. This would effectively create separate, identical address pools for each of the VLANs, though I'm not sure how a router would differentiate between the two when interVLAN communication is attempted. I'm a little bit unsure about the pros/cons of this configuration..
A single subnet across multiple VLANs, also called Transparent subnet gatewaying (RFC 1027) is a somehow archaic approach. It uses Proxy ARP, but proxy ARP has it's own set of problems.
Multiple subnets to a single VLAN (connected via a switch):
Hosts across both subnets would receive layer 2 broadcasts (such as ARP), but would ignore traffic lacking an IP that targets them.
Question: Would I be able to communicate across subnets without a
layer 3 device if I could manually insert a destination MAC address in
the frame header?
You will need to replace the MAC address, and need to recalculate FCS over the whole frame, else the switch will reject it as a damaged frame. This must happen after your ethernet driver does this.
A single subnet across multiple VLANs:
Broadcast traffic would be isolated to the individual VLANs. This would break ARP, as a host targeting another machine in the same
subnet (but unknowingly in another VLAN) would send out an ARP
request that would never be responded to. This would effectively
create separate, identical address pools for each of the VLANs,
though I'm not sure how a router would differentiate between the two
when interVLAN communication is attempted. I'm a little bit unsure
about the pros/cons of this configuration..
Why would we ever want to do this?
Hosts in the same subnet would not be able to communicate with each other. Most routers will not let you assign the same network to multiple interfaces, unless they are bridged interfaces, in which case, you haven't accomplished anything except sending the traffic the long way around.
Some switches have something similar to this, called Private VLANs, where hosts can only communicate with a gateway. This is a security feature used in some situations.
can one location have more than 1 IP address? I have 2 IP addresses and need to know if they originate from the same source. Thanks
You're missing clear definitions of "location" and "source" in your question but lets go with the assumption that you mean "physical machine" for both (as it's obvious that multiple machines will have different addresses and that a single machine can change its IP address over time).
In that case, the answer is yes. The operating system may bind as many IP addresses to a physical network port (and a single MAC -- the physical addressing used by Ethernet) as it wishes.
Binding multiple IP addresses was the standard way of doing "virtual web hosting" before HTTP/1.1 arrived with the "Host" header. The provider would use DNS to map different host names to different IPs on the same network (usually the same subnet as well) and then assign all of them to the same interface. The webserver would get address information from an incoming connection and based on the local IP address would know which virtual host was being accessed.
This led to a higher-than-typical use of public IP addresses but the practice is now gone with the proliferation of HTTP/1.1.
I'm not sure how Windows presents it, but Linux will present a physical interface with multiple IP addresses as multiple logical interfaces such as "eth0", "eth0:1", "eth0:2", etc. Each logical interface has a unique IP address even though they share the same physical interface.
This is hard to tell. Especially it is hard to tell if these IP addresses are from different times. Today I may have a different IP address than tomorrow.
Even they are from the same time, a load balancer im my internal equipment might send my packets over the fail-over line if the 1st one is overloaded or broken down.
One network adapter normally has only one IP address at once and a typical end user only has one network connection active, but even then the IP address can change. The user could switch from wireless to wired and back or a power outage might reset the ISP's DHCP server (assigning everyone a new IP address).
If you want to identify the user even if his IP address changes, you need to identify the user by his session id, stored in cookies. As two users can have the same IP address (a whole company could be behind a NAT), you should never rely on IP addresses for identification.
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.