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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?

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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'.

A 0.0.0.0/1 prefix matches any address from 0.0.0.0 to 127.255.255.255 (practically 1.0.0.0 to 126.255.255.255; 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 128.0.0.0/1.

0.0.0.0/8 and 0.0.0.0/32 specifically are reserved by RFC 1122 section 3.2.1.3.

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    So the CIDR representation - 0.0.0.0/1 is practically used to represent an IP addresses range - 1.0.0.0 to 126.255.255.255, is that right?
    – Deepak
    Nov 14, 2022 at 14:09
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    Yes, I've made that clearer in the answer.
    – Zac67 ♦
    Nov 14, 2022 at 14:33
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    0.0.0.0/0 is the only prefix that matches any address, so yes.
    – Zac67 ♦
    Nov 14, 2022 at 14:39
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    I've also seen the CIDR pair 0.0.0.0/1 and 128.0.0.0/1 being using to apply a reversible route to 0.0.0.0/0. This is typically done on vpn-up/down.sh scripts.
    – Aron
    Nov 15, 2022 at 3:36
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    @user253751 With a prefix shorter than /24, the .0 address might not be anything special. It's the all-host-bits-zero address that's largely unusable with IPv4.
    – Zac67 ♦
    Nov 15, 2022 at 7:37
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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.)

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  • I would add that the pair of CIDRs allow you to "override" 0.0.0.0/0 without "overwriting" you 0.0.0.0/0 route.
    – Aron
    Nov 15, 2022 at 3:38
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    That's what the second paragraph says.
    – Ricky
    Nov 15, 2022 at 23:49
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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.

  1. It's the CIDR notation for the block of addresses that were formerly known as class A (class B is 128.0.0.0/2, class C is 192.0.0.0/3 class D is 224.0.0.0/4 and class E is 240.0.0.0/4) .
  2. 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 128.0.0.0/1 pointed at another link to use each link for (very roughly) half of destinations.
  3. Some VPN software, notably openvpn, uses a pair of routes for 0.0.0.0/1 and 128.0.0.0/1 to effectively override the default gateway without modifying or removing it.
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