While subnetting might be easy enough to grasp as a concept, it can be a bit involved, and even mind-boggling in part due to the required manipulations of binary numbers. For additional information on subnetting, see RFC 1817, and RFC 1812. Traditionally, in an Ethernet network, it is very common for all nodes on a segment to see all the packets transmitted by all the other nodes on that segment, which introduces collisions, and the resulting retransmissions under heavy traffic loads. The most common reason for subnetting IP networks is to control network traffic. It essentially organizes the hosts into logical groups, and provides for improving network security and performance. In the second scenario the router can only communicate with those ip addresses that fall within the /24 subnet, it can't communicate with any hosts with ip addresses above 192.168.1.255 because those ip addresses fall outside of 192.168.1.0/24, which is the routers subnet in the second scenario.Subnetting an IP network allows for the flow of network traffic to be segregated based on a network configuration. In the first scenario all the hosts can communicate with the router because the router is using a /20 subnet mask BUT it's ip address falls within both subnets from the perspective of hosts with either subnet mask. That's why the first scenario works but the second scenario doesn't. Any host with an ip address in the /24 range would appear to belong to the local subnet to any of the hosts and more importantly in this scenario, to the router, BUT the hosts in the /24 subnet will only be be able to communicate with other hosts whose ip addresses also fall within the 192.168.1.0-192.168.1.255 range. A Host doesn't know what subnet mask any other host is using, it only knows it's own subnet mask and uses that to determine which ip addresses are local and which are not. It exists in both the /20 and the /24 subnets. With a /20 subnet mask your ip address range is 192.168.0.0-192.168.15.255 (including the network and broadcast addresses).įrom the perspective of hosts with either subnet mask, 192.168.1.100 is a valid local address. With a /24 subnet mask your ip address range is 192.168.1.0-192.168.1.255 (including the network and broadcast addresses). This answer assumes that your router has an ip address of 192.169.1.100 (or any ip address between 192.168.1.0-255). I'm not sure whether this is worth mentioning but the DHCP server is running ubuntu dhcp server. My question to this is: If the previous scenario works with both subnets. The devices served dynamic IPs with subnet mask /20 cannot access the network. The outcome? The devices with static IPs with subnet mask /24 work. This time the router's subnet mask remained at /24 and I only modified the DHCP server's subnet mask to /20.
![blank subnet mask table blank subnet mask table](https://2.bp.blogspot.com/-wo6YfV64lng/VZGJWTLTQrI/AAAAAAAAAJs/ba2C6umIf98/w1200-h630-p-k-no-nu/IPv4%2BSubnet.png)
My question to this first scenario is: Aren't all devices supposed to have the same subnet mask in order to work? Why are the workstations with static ip addresses and a netmask of /24 still working?
![blank subnet mask table blank subnet mask table](https://2.bp.blogspot.com/-PC93_LC7D1o/WirgVxb-_II/AAAAAAAA7Ys/SrmowOZldikYsQMgUovNL6rxfd8-YtYdQCLcBGAs/s1600/subnetting-quick-reference-chart.png)
I connected a few workstations and wireless devices to the network and all of them got served the correct range, with /20 and are able to access the network. However, I did not change the subnet mask of any of the workstations with static ips and they stayed at /24.
![blank subnet mask table blank subnet mask table](https://folderku.web.id/wp-content/uploads/2021/06/notasi-cidr.png)
This was the first scenario, I changed the router's subnet mask to /20 to match that of my dhcp server.
![blank subnet mask table blank subnet mask table](https://3.bp.blogspot.com/-4vGe125qN1k/WdYijTQqIUI/AAAAAAAAA-o/57K-hV1dPO84-FJKJHAynjyDYTDfuO1_gCLcBGAs/s1600/subnet%2Bmask%2Bfor%2Bdifferent%2Bip%2Bclassess.png)
I was trying to increase the number of ip addresses available from my dhcp server by changing the netmask from /24 to /20. Ran into something funny today and am still scratching my head on why this happened.