I ran an nslookup from my system at work and got the following response:
Server: WHATEVER
Address: xxx.xxx.xxx.xxx
Non-authoritative answer:
Name: mcmaster.com
Address: 209.64.25.230
Server: whatever
Address: xxx.xxx.xxx.xxx
Non-authoritative answer:
Name: a1843.b.akamai.net
Addresses: 23.66.232.17
23.66.232.26
Aliases: www.mcmaster.com
www.mcmaster.com.edgesuite.net
My initial thought was whoever owns DNS for Mcmaster simply didn't have www as a valid response. I know I forward requests to www.whateverdomainirun.com to whateverdomainirun.com, but, seeing different DNS for the two surprised me.
When I navigate to http://mcmaster.com I get forwarded to the www. When I run a tracert on mcmaster.com it fails with Destination net unreachable. It looks to me like this company has put their site in Akamai's hands and either that's recent so your local DNS servers haven't updated and you could try flushing the cache and doing it again, or, in some way you're blocking requests to Akamai for some reason on your firewall.
Best and quick guess for you and it may be wildly wrong :).