IP addressing and subnetting is a common interview subject. I assert that memorizing these things is useful for learning the concepts but ultimately futile in that it is time consuming and inefficient use of engineering time when tools can be utilized to accomplish the same goals in less time with fewer errors. Honestly, I gave up doing this kind of work manually around 10 years ago and have never regretted it, and in actuality, I’d probably struggle to do it at this point because it’s a repetitive process better suited by code. In the old days, subnetting IPv4 manually was a badge of honor (and one that I always hated), but I learned it because I needed to know it for cert tests and daily work. However, once I started doing IPv6 around 2001, it became clear that doing this kind of thing by hand was consuming more time than it needed to. Enter UNIX tools. HEX Hex isn’t really a tool as much as it’s a hack for your shell.  Remember the post on dotfiles? This is something that can go right into your .bashrc and allows for the quick and easy translation of decimal to hexidecimal, which is very useful for IPv6 dual stacking because [in my opinion] the appropriate addressing scheme is to match the last octet based on hex and not numerically. So, to do that one needs to be able to easily convert the last octet quickly and easily.  Adding this to your .bashrc will accomplish this:``` alias hex=‘printf “%x\n”’

(~) desktop $ hex 199
c7

If you’re using static addresses or dhcpv6 with static addressing, you can match the last octet properly.``` 10.143.27.¹⁹⁹⁄₂₇ 2001:DB8:1b::c7/120

(~) desktop $ sipcalc 2001:DB8:1b::c7/120
-\[ipv6 : 2001:DB8:1b::c7/120\] - 0
``````
\[IPV6 INFO\]
Expanded Address - 2001:0db8:001b:0000:0000:0000:0000:00c7
Compressed address - 2001:db8:1b::c7
Subnet prefix (masked) - 2001:db8:1b:0:0:0:0:0/120
Address ID (masked) - 0:0:0:0:0:0:0:c7/120
Prefix address - ffff:ffff:ffff:ffff:ffff:ffff:ffff:ff00
Prefix length - 120
Address type - Aggregatable Global Unicast Addresses
Network range - 2001:0db8:001b:0000:0000:0000:0000:0000 -
 2001:0db8:001b:0000:0000:0000:0000:00ff -

And for IPv4: (~) desktop $ sipcalc 10.143.27.199/27 -\[ipv4 : 10.143.27.199/27\] - 0 \[CIDR\] Host address - 10.143.27.199 Host address (decimal) - 177150919 Host address (hex) - A8F1BC7 Network address - 10.143.27.192 Network mask - 255.255.255.224 Network mask (bits) - 27 Network mask (hex) - FFFFFFE0 Broadcast address - 10.143.27.223 Cisco wildcard - 0.0.0.31 Addresses in network - 32 Network range - 10.143.27.192 - 10.143.27.223 Usable range - 10.143.27.193 - 10.143.27.222   Notable mention: Web tools are also useful and are becoming more prolific than the UNIX tools, but I will assume that you’re probably already loged into a UNIX system like a jump box or bastion host anyway and the tools are faster and thinner in that environment. That said, here are some useful web tools: ipcalc has the web interface to their tool. This site has a v4 and v6 calculator that works well and looks a lot like the output of sipcalc.