When configuring MSSQL clustering, encountering IP address conflicts is a common yet frustrating scenario. The situation where pinging 10.40.1.205 returns a response from 10.40.59.69 indicates one of several potential network configurations at play:

Pinging 10.40.1.205 with 32 bytes of data:
Reply from 10.40.59.69: bytes=32 time=1ms TTL=128
Reply from 10.40.59.69: bytes=32 time=1ms TTL=128

1. NAT or Proxy Configuration: Network Address Translation might be redirecting traffic. Check with:

tracert 10.40.1.205
route print

2. Duplicate IP Detection: MSSQL's cluster validation might be detecting ARP anomalies. Verify with:

arp -a 10.40.1.205
netsh interface ipv4 show neighbors

3. Subnet Misconfiguration: The responding host might be in a different VLAN. Validate subnet masks:

ipconfig /all | findstr "Subnet Mask"

For immediate cluster setup, consider these concrete steps:

1. Perform thorough IP scanning of the network segment:

   nmap -sn 10.40.1.0/24

2. Configure cluster network validation override (if absolutely certain):

   New-Cluster -Name SQLCluster -Node Node1,Node2 -StaticAddress 10.40.1.205 -IgnoreNetwork 10.40.1.0/24

3. Validate network configuration through PowerShell:

   Test-Cluster -Node Node1 -Include "Inventory","Network","Storage"

Modern virtualized environments often exhibit this behavior due to:

• Hyper-V virtual switches with MAC address spoofing enabled
• SDN overlays creating virtual network segments
• Load balancers intercepting and redirecting traffic

For comprehensive validation, capture network traffic during cluster validation:

netsh trace start capture=yes scenario=NetConnection tracefile=C:\temp\nettrace.etl
# Run cluster validation
netsh trace stop

During MSSQL cluster configuration, encountering IP address conflicts is more common than many DBAs realize. What makes your case particularly interesting is that while 10.40.1.205 appears available (per network admin), practical tests reveal:

C:\>ping 10.40.1.205
Reply from 10.40.59.69: bytes=32 time=1ms TTL=128

Several network conditions could explain this behavior:

• Proxy ARP Configuration: A router might be answering for unreachable hosts
• Duplicate IP Detection: Some systems respond to pings during IP conflict checks
• Load Balancer Interception: VIP configurations might redirect traffic
• Ghost IP: Former cluster node leaving ARP cache entries

Run this to gather comprehensive network data:

$targetIP = "10.40.1.205"
$results = @{
    'PingResult' = Test-Connection $targetIP -Count 2
    'ARPEntry' = Get-NetNeighbor -IPAddress $targetIP -ErrorAction SilentlyContinue
    'Route' = Get-NetRoute -DestinationPrefix $targetIP -ErrorAction SilentlyContinue
    'PortTest' = Test-NetConnection $targetIP -Port 1433
}
$results | ConvertTo-Json -Depth 5 | Out-File "IP_Diagnostics.json"

Before proceeding with cluster setup, validate IP availability through SQL Server:

-- Check for existing IP registrations
SELECT * FROM sys.dm_os_cluster_nodes
WHERE ip_address = '10.40.1.205'

-- Alternative T-SQL ping test
DECLARE @result INT
EXEC @result = xp_cmdshell 'ping -n 2 10.40.1.205'
IF @result = 0
    PRINT 'IP responds to ping'
ELSE
    PRINT 'IP appears free'

When basic tools don't reveal the issue, packet-level inspection helps:

netsh trace start capture=yes tracefile=C:\Temp\net_trace.etl
ping 10.40.1.205
netsh trace stop

Analyze the ETL file with Message Analyzer or Wireshark, focusing on:

• ARP request/response patterns
• ICMP echo request destination vs reply source
• Any unexpected protocol handling

In managed environments, additional factors may apply:

# Cisco IOS check for proxy ARP
show ip interface | include Proxy
show arp 10.40.1.205

# Windows NLB verification
nlbmgr /hostlist

Follow this systematic approach:

1. Clear ARP cache: arp -d *
2. Verify switch port assignments
3. Check DHCP reservations
4. Inspect DNS PTR records
5. Review network device configurations