The key difference between 0.0.0.0/0 and 0.0.0.0/1 lies in their prefix lengths and the resulting address ranges they cover:

// Address range calculations
0.0.0.0/0  = Match ALL IPv4 addresses (0.0.0.0 - 255.255.255.255)
0.0.0.0/1  = Covers first half of IPv4 space (0.0.0.0 - 127.255.255.255)
128.0.0.0/1 = Covers second half (128.0.0.0 - 255.255.255.255)

While 0.0.0.0/0 is commonly used as a wildcard (e.g., in firewall rules), 0.0.0.0/1 has specific networking use cases:

# Example firewall rule splitting traffic
iptables -A INPUT -s 0.0.0.0/1 -j ACCEPT_FIRST_HALF
iptables -A INPUT -s 128.0.0.0/1 -j ACCEPT_SECOND_HALF

Network engineers might use 0.0.0.0/1 plus 128.0.0.0/1 instead of 0.0.0.0/0 for:

• BGP route filtering (more specific controls)
• Load balancing between network paths
• Testing network segmentation
• Creating "catch-all" routes with priority ordering

Here's how you might implement this in a routing configuration:

# Cisco IOS example
ip route 0.0.0.0 0.0.0.0 192.168.1.1
ip route 0.0.0.0 128.0.0.0 192.168.2.1

# Linux routing table equivalent
route add -net 0.0.0.0/1 gw 192.168.1.1
route add -net 128.0.0.0/1 gw 192.168.2.1

When working with IP routing and firewall configurations, two special CIDR notations often appear: 0.0.0.0/0 and 0.0.0.0/1. While they might seem similar at first glance, their behavior differs significantly in network operations.

0.0.0.0/0 represents the entire IPv4 address space. This is the most inclusive CIDR notation possible, matching every possible IP address from 0.0.0.0 to 255.255.255.255.

# Linux route command example
route add -net 0.0.0.0/0 gw 192.168.1.1

# iptables rule matching all traffic
iptables -A INPUT -s 0.0.0.0/0 -j DROP

0.0.0.0/1 covers exactly half of the IPv4 address space - specifically addresses from 0.0.0.0 to 127.255.255.255 (the former Class A network range). The /1 prefix length means only the first bit is fixed (as 0), leaving 31 bits variable.

# Practical use in AWS Security Groups
{
  "IpProtocol": "tcp",
  "FromPort": 80,
  "ToPort": 80,
  "IpRanges": [{"CidrIp": "0.0.0.0/1"}]
}

Network engineers might use 0.0.0.0/1 in combination with 128.0.0.0/1 to:

• Implement traffic splitting between two ISPs
• Create more specific routing policies
• Work around limitations in some cloud providers' route tables

Here's how you might use both notations in a BGP configuration:

router bgp 64512
 neighbor 192.0.2.1 remote-as 64513
 !
 address-family ipv4
  network 0.0.0.0/1
  network 128.0.0.0/1
  neighbor 192.0.2.1 route-map SET-PREF out
 exit-address-family
!
route-map SET-PREF permit 10
 match ip address prefix-list HALF-SPACE
 set local-preference 200
!
ip prefix-list HALF-SPACE seq 5 permit 0.0.0.0/1

When used in firewall rules, 0.0.0.0/1 provides more granular control than 0.0.0.0/0:

# Less permissive alternative to 0.0.0.0/0
ufw allow from 0.0.0.0/1 to any port 22
ufw allow from 128.0.0.0/1 to any port 22

This two-rule approach can be useful when firewall systems don't properly handle 0.0.0.0/0 or when you need to apply different policies to each half of the address space.