The error message connect: No route to host when attempting IPv6 connectivity on macOS typically indicates one of these scenarios:

• The system isn't properly selecting the outgoing interface for link-local addresses
• Missing scope identifier for link-local addresses
• Firewall blocking IPv6 communications

The correct command format when dealing with link-local IPv6 addresses requires specifying the scope identifier (interface index):

telnet fe80::xxxx:xxxx:xxxx:xxxx%en1 80

Where en1 should be replaced with your actual network interface (find it using ifconfig or networksetup -listallhardwareports).

Before troubleshooting, verify your IPv6 configuration:

ifconfig | grep inet6
netstat -rn -f inet6

Example output showing scope identifiers:

en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    inet6 fe80::1a2b:3c4d:5e6f:1234%en1 prefixlen 64 scopeid 0x5

When telnet fails, try these diagnostic tools:

# Test basic connectivity
ping6 -c 4 -I en1 fe80::xxxx:xxxx:xxxx:xxxx%en1

# Test port connectivity
nc -6 -v -z fe80::xxxx:xxxx:xxxx:xxxx%en1 80

# Test with curl (for HTTP services)
curl -g -6 "http://[fe80::xxxx:xxxx:xxxx:xxxx%en1]/"

Check macOS application firewall settings:

sudo /usr/libexec/ApplicationFirewall/socketfilterfw --listapps

If needed, temporarily disable the firewall for testing:

sudo pfctl -d

Here's a full sequence that should work when testing a web server:

# Find your interface
networksetup -listallhardwareports

# Ping test
ping6 -I en1 fe80::xxxx:xxxx:xxxx:xxxx%en1

# Telnet test (requires %interface scope)
telnet fe80::xxxx:xxxx:xxxx:xxxx%en1 80

# Alternative using nc
nc -6 fe80::xxxx:xxxx:xxxx:xxxx%en1 80

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When attempting to telnet to an IPv6 link-local address (fe80::/10) on Mac OS X, you might encounter the "connect: No route to host" error despite successful ping6 connectivity. This behavior differs from Windows where the same telnet command works.

The fundamental difference lies in how Mac OS X handles IPv6 link-local addresses:

# This works because ping6 explicitly specifies the interface
ping6 -I en1 fe80::a00:27ff:fe12:3456

# This fails because telnet can't determine the outgoing interface
telnet -6 fe80::a00:27ff:fe12:3456 80

Mac OS X (and BSD-derived systems) require explicit interface specification for link-local IPv6 communication. Unlike Windows, the interface isn't automatically deduced. There are three approaches to resolve this:

1. Using the %interface Notation

telnet fe80::a00:27ff:fe12:3456%en1 80

2. Using the -S Flag (BSD-style)

telnet -S fe80::a00:27ff:fe12:3456%en1 80

3. Alternative Tools Approach

If standard telnet doesn't work, try alternative tools that better handle IPv6:

# Using netcat (nc)
nc -6 fe80::a00:27ff:fe12:3456%en1 80

# Using curl for HTTP testing
curl -g "http://[fe80::a00:27ff:fe12:3456%en1]/"

Ensure your interface is properly configured for IPv6:

# Check interface status
ifconfig en1 | grep inet6

# Verify IPv6 routing table
netstat -rn -f inet6

# Test global connectivity
ping6 2606:4700:4700::1111

Mac's pf firewall might block IPv6 connections. Check with:

sudo pfctl -s all | grep inet6

Temporarily disable firewall for testing:

sudo pfctl -d
# Remember to re-enable after testing: sudo pfctl -e

To modify IPv6 behavior system-wide:

# Check current IPv6 settings
sysctl net.inet6.ip6

# Enable IPv6 forwarding (if needed)
sudo sysctl -w net.inet6.ip6.forwarding=1

For deeper troubleshooting, use these diagnostic commands:

# Packet capture on specific interface
sudo tcpdump -i en1 ip6

# Detailed socket connection attempt
sudo dtrace -n 'syscall::connect*:entry { printf("%s %s", execname, copyinstr(arg1)); }'