When expanding a subnet mask from /24 (255.255.255.0) to /16 (255.255.0.0) in a 192.168.0.0 network, you're fundamentally altering how devices interpret network boundaries. In your current setup:

Current configuration:
Network: 192.168.0.0
Mask: 255.255.255.0
Broadcast: 192.168.0.255
Hosts: 192.168.0.1 - 192.168.0.254

After the change:

New configuration:
Network: 192.168.0.0
Mask: 255.255.0.0
Broadcast: 192.168.255.255
Hosts: 192.168.0.1 - 192.168.255.254

Three critical systems will be affected differently:

• DHCP Clients: Will automatically adopt the new subnet mask upon lease renewal
• Statically Configured Devices: Will require manual updates to maintain proper routing
• Network Services: May need ACL/rule updates (printers, NAS, etc.)

The most dangerous consequence isn't immediately visible. Devices with the old /24 mask won't ARP for addresses outside 192.168.0.x, creating communication black holes. This Python snippet simulates the issue:

import scapy.all as scapy

def check_arp_reachability(target_ip, interface):
    # This will fail for 192.168.2.x if local mask is /24
    ans, unans = scapy.arping(target_ip, iface=interface, verbose=False)
    return len(ans) > 0

# Test case:
print(check_arp_reachability("192.168.2.1", "eth0"))  # Returns False if local mask is /24

For ISC DHCPD, you'd modify your config like this:

subnet 192.168.0.0 netmask 255.255.0.0 {
    option routers 192.168.0.1;
    option subnet-mask 255.255.0.0;
    
    # Original range remains unchanged
    pool {
        range 192.168.0.100 192.168.0.200;
    }
    
    # New printer range
    pool {
        range 192.168.2.10 192.168.2.50;
        deny unknown-clients;
    }
}

Follow this phased approach to minimize disruption:

1. Update all static devices first (servers, network gear)
2. Implement DHCP changes during low-usage hours
3. Monitor ARP tables for inconsistencies:

   # Linux command to monitor ARP anomalies
   watch -n 5 'arp -an | grep -v "192\.168"'

4. Update firewall rules to reflect new IP groupings

For precise device grouping, consider DHCP class matching:

class "Printers" {
    match if substring(hardware, 1, 3) = 00:01:95;  # Example printer OUI
}

subnet 192.168.0.0 netmask 255.255.0.0 {
    # ... other config ...
    
    pool {
        range 192.168.2.10 192.168.2.50;
        allow members of "Printers";
    }
}

When expanding your subnet mask from 255.255.255.0 (/24) to 255.255.0.0 (/16), you're fundamentally altering how devices interpret network boundaries. This affects:

• Broadcast domain expansion from 254 to 65,534 usable addresses
• ARP traffic behavior changes
• Routing table impacts for multi-subnet environments

For a Windows DHCP server, you'll need to adjust both the scope options and server options:

# PowerShell to modify DHCP scope
Set-DhcpServerv4Scope -SubnetMask 255.255.0.0 -ScopeId 192.168.0.0
Set-DhcpServerv4OptionValue -OptionId 3 -Value 192.168.0.1 -ScopeId 192.168.0.0

Key considerations:

• Existing DHCP leases won't automatically update - clients must renew
• Option 1 (Subnet Mask) and Option 3 (Router) must be consistent
• Reservations need verification for correct mask application

Any device with manually configured IPs requires individual updates:

# Linux network config example (Debian/Ubuntu)
auto eth0
iface eth0 inet static
    address 192.168.1.10
    netmask 255.255.0.0
    gateway 192.168.0.1

Critical systems needing attention:

• DNS servers
• Default gateways
• VPN endpoints
• Network storage devices

For your planned IP range organization:

# Example DHCP scope partitioning
Add-DhcpServerv4ExclusionRange -ScopeId 192.168.0.0 -StartRange 192.168.2.1 -EndRange 192.168.2.254
Add-DhcpServerv4Reservation -ScopeId 192.168.0.0 -IPAddress 192.168.2.100 -ClientId (MAC) -Name "Printer01"

Best practices for range allocation:

Be prepared to address:

• Firewall rules referencing /24 subnets
• ACLs on network equipment
• Monitoring systems with subnet-based alerts
• VPN configurations with split-tunneling

Post-migration validation steps:

1. Test communication between old (.0.x) and new (.2.x) ranges
2. Verify DNS resolution across subnets
3. Check multicast/broadcast-dependent services
4. Validate VLAN configurations if applicable