Some basics you need to know first
111.221.74.0/24
isn't a valid IP because an IP with an ending zero is the network identifier
A following backslash with a number is an alternative notation for a subnet mask
111.221.74.0 <- network identifier
255.255.255.0 <- subnet mask
This subnet mask is in decimal writing. If we convert both numbers to binaries, you will see that there are 8 zeros and 24 ones. That's where the /24
came from
0110 1111.1101 1101.0100 1010.0000 0000 <- network identifier
1111 1111.1111 1111.1111 1111.0000 0000 <- subnet mask
|______ network prefix _____| |_hosts_|
The zeros define (mask) the valid first and last IP for hosts of that network
0110 1111.1101 1101.0100 1010.0000 0001 <- first valid IP address for hosts
0110 1111.1101 1101.0100 1010.1111 1110 <- last valid IP address for hosts
The 1111 1111
(=255) is reserved for the broadcast address
0110 1111.1101 1101.0100 1010.1111 1111 <- broadcast address
Back to your router problem
A look at the Cisco DPC3825 manual (site 60) reveals that your router wants the start and end IP to block a whole range of IP addresses. For 111.221.74.0/24
the start/end address would be 111.221.74.1
and 111.221.74.254
.
Edit
You cannot enter these skype IPs under "Access Restrictions > IP Address Filtering". This subpage is only for outbound traffic respectively IPs from local hosts which are connected to your router. Usually they begin with 192.168.xxx.yyy
. For example, to block your child's laptop you would enter (start:) 192.168.1.100 - (end:) 192.168.1.100. The cisco router checks if an entered IP is within the range of 192.168.xxx.yyy and since it's not he states "invalid IP".
You want to block inbound traffic for specific IPs. As far as I can tell, this seems impossible for your Cisco DPC3825 (which runs a Linksys software btw). Your best chance is under "Access > Basic Rules" since your Skype block guide says
Currently there are 16 DNS entries from dsn0.d.skype.net to dsn16.dsn.skype.net.
Try to enter URLs instead of IPs
dsn0.d.skype.net
dsn1.d.skype.net
...
dsn16.d.skype.net
You may reach the maximum of blocking rules. If this happens, try if wildcards (*, ?) are allowed: dsn*.d.skype.net