I know that
0.0.0.0/0 represents a default route.
But I would like to understand what does
0.0.0.0/1 represents? Is it even a valid thing in networking?
0.0.0.0 is no valid IP address, regardless of prefix length.
0.0.0.0/0 as prefix matches any address, so it's used for the default route. Also,
0.0.0.0 is the unspecified address used in many APIs, indicating 'all local addresses'.
0.0.0.0/1 prefix matches any address from
0.0.0.0/8 is reserved for IANA special use and generally invalid,
127.0.0.0/8 is reserved for local loopback) - if that makes sense for you. I've seen at least one instance of crude 'load balancing' with two WAN uplinks where one was used for
0.0.0.0/1 and the other for
0.0.0.0/32 specifically are reserved by RFC 1122 section 220.127.116.11.
Zac is muddying the waters by excluding 0.0.0.0/8 and 127.0.0.0/8. While addresses in those two prefixes should never appear on the wire, 0.0.0.0/1 does cover them. The slash notation works the same for zero and one as it does for any number from 0 to 32, inclusive. "The number of bits that matter." One bit means 0-127, and 128-255; since you stated 0.0.0.0/1, it's the former [addresses 0.0.0.0 through 127.255.255.255, or in hex 00000000 - 7FFFFFFF]
It's a common trick of VPN software to use 0/1 and 128/1 routes as more specific than the (0/0) default route, without having to muck with other processes (eg. DHCP - if you mess with the routes DHCP installed, it will reinstall them when the lease renews.)
(But otherwise, yes, 0.0.0.0 is not a legal interface address.)
0.0.0.0/1 is not an IP address per-se, it's the combination of an IP address and a prefix length in CIDR notation. It represents the range of addresses from 0.0.0.0 to 127.255.255.255.
There are a few places it might crop-up.
- It's the CIDR notation for the block of addresses that were formerly known as class A (class B is 18.104.22.168/2, class C is 192.0.0.0/3 class D is 22.214.171.124/4 and class E is 240.0.0.0/4) .
- As Zac97 suggests it may be used as a crude approach to load balancing, with 0.0.0.0/1 pointed at one link and 126.96.36.199/1 pointed at another link to use each link for (very roughly) half of destinations.
- Some VPN software, notably openvpn, uses a pair of routes for 0.0.0.0/1 and 188.8.131.52/1 to effectively override the default gateway without modifying or removing it.