Subnet mask is not part of normal packet exchange so you will not see that in sniffer captures anyway unless you have a classless routing protocol running that advertise a mask with routing updates or you have DHCP replies from a DHCP server. Additionally, users on your LAN have to know the subnet mask as they are normally served by the router as a gateway for that LAN.
What is your intention? Maybe if you explain what you want to achieve we can suggest an alternate way of doing it.
As Atif has quite rightly said, the network mask is not sent out in the packet. Have a look at the attachment - it's a screen shot of a sniffer with a ping from my pc to cisco.com.
There is no need for network masks in the packet header as that's not how ip works. What will happen is:
You pc wants to know where an IP address lives. It will look at it's own routing table ("route print" in dos). If the address is not in it's own subnet, then it will send it to the default gateway (which is why 2 default gateways on a server causes trouble). It will then send a packet to the default gateway to send it to the destination. This is usually a router and should have a route to it in it's routing table.
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
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