Tjyylli

I am very poor with Computer Networks, even though I took class in my grad school.

I keep forgetting about subnets and masks and what not!

Currently, I am playing with Docker and Erlang and was trying to do distributed programming.

I started 4 docker containers

(gru@172.17.0.63) # also known as Server 

(minion1@172.17.0.61)

(minion2@172.17.0.64)

(minion3@172.17.0.65)

where numbers after @ are their respective IPs taken from ifconfig command.

Now given these IP addresses, how can I tell if they are on same LAN/network

If question is asked incorrectly or is stupid, my apologies, but I am stuck!

Thanks a lot

You need to know the subnet mask. The subnet mask tells you which bits of the address represent the subnet, and which bits represent the host within the subnet. If you subnet is a /24 (255.255.255.0), then any address whose first three octets match would be in the same subnet.

In your example, if your subnet mask is a /26 or longer (255.255.255.192 or higher), then those addresses would not be on the same subnet, because 63 would be the broadcast address of one subnet, and 64 would the the "network" address (0 for the host number) of another subnet. So those addresses probably wouldn't even work reliably as unicast host addresses if your subnet mask was /26 or longer.

The IP alone is not enough to determine if it is on the same network. You need to use the subnet mask as well.

An IP address is made up of four 8-bit octets; an octet is a sequence of 8 binary bits. Each bit therefore can be either a 0 or a 1, so each octet can represent 2^8 different values, which is 256 (0-255). Since there are four of these octets, there can be a combination of addresses equaling 28+8+8+8, or 232, which is 4,294,967,296 possible addresses. Each binary bit has a value: 128, 64, 32, 16, 8, 4, 2, 1, so depending on whether that binary bit is switched on or off, we either add or don’t add the value for that bit.

The mask indicates the amount of addresses that we are ignoring. Basically, where there are 1s, there are addresses that are not a part of the group our address is in. For a /24 network (or network with mask of 255.255.255.0), it means that out of 2^32 possible addresses, we are subtracting 2^24 addresses, so we are left with 2^8 addresses in our group, or 256 of them.

So if you had an address of 172.17.0.0/24, it means that you would have 256 addresses in that network, which would be 172.17.0.0-172.17.0.255, though some would be reserved as the network address (172.17.0.0) and one would be reserved as the broadcast address (172.17.0.255) and you'd also need a gateway (usually 172.17.0.1).