What's the meaning of CIDR notation like "1" or "4/6" in mac OS X netstat output

Question

I ran netstat -nr in my Mac OS X and see the following output. What does the CIDR notation like 1 , /2/7, 4/6 mean?

Destination        Gateway            Flags           Netif Expire
default            172.20.10.1        UGScg             en0
default            172.20.10.1        UGScIg            en6
1                  198.18.0.1         UGSc            utun3
2/7                198.18.0.1         UGSc            utun3
4/6                198.18.0.1         UGSc            utun3

Edit: Normally I see notations like 1.0.0.0/24 (with the four dots and a slash followed by subnet mask), but what does it mean when it has no dot nor subnet mask? I can't seem to find an explanation for it.