I just noticed something interesting on my OpenWRT home screen, about the DHCP IPv4 address I received from my ISP:
What does the "/21" on the address mean? Does that mean they allocated me 8 IPv4 addresses for my use?
IPv4 addresses are made up of 4 octets meaning there are a possible 32 bits in total.
CIDR notation uses "/" slash and a number (called suffix) that indicates how many bits are used to identify the network portion of the address.
In your case 21 bits are used to identify the network and the remaining 11 bits are used to identify the hosts.
This means that the available host addresses in that IP subnet are 2046 (from X.138.176.1 to X.138.183.254), but they are definitely not for your use (except one) 
.
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No, it just means that all devices on the X.138.176.1 to X.138.183.254 range can be reached without having to go through the gateway.
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