In CentOS/RHEL systems, you can assign multiple IP addresses to a single physical interface using interface aliases (e.g., eth0:0). This is particularly useful when you need to host services on different subnets without additional hardware.


Here's how to properly configure eth0:0 on a different subnet while maintaining the primary eth0 configuration:

Primary Interface (eth0):
# /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
IPADDR=192.168.91.250
NETMASK=255.255.255.0
GATEWAY=192.168.91.1
ONBOOT=yes

Alias Interface (eth0:0):
# /etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE=eth0:0
BOOTPROTO=static
IPADDR=10.10.191.210
NETMASK=255.255.255.0
# Notice no gateway here - more on this below
ONPARENT=yes


The critical consideration is routing. While you can technically specify different gateways in each ifcfg file, this often causes routing conflicts. Instead:

1. Keep only the primary gateway in the main interface
2. Add specific routes for the secondary subnet:

# /etc/sysconfig/network-scripts/route-eth0:0
10.10.191.0/24 via 10.10.191.254 dev eth0:0


For /etc/resolv.conf, you have two approaches:

Option 1: Use DNS servers that can resolve both networks
nameserver 8.8.8.8
nameserver 8.8.4.4

Option 2: Use conditional forwarding (requires dnsmasq or similar)
server=/10.10.191.0/24/10.10.15.161
server=/192.168.91.0/24/192.168.91.1


After configuration:
# ifup eth0:0
# ip addr show
# route -n
# ping -I eth0 192.168.91.1
# ping -I eth0:0 10.10.191.254

Common issues to check:
- Make sure ONPARENT=yes is set in alias configurations
- Verify no conflicting gateways
- Check SELinux context if connections fail




In CentOS 5.7 (and most Linux distributions), you can indeed assign multiple IP addresses to a single physical interface using interface aliases. The key requirement is that these IPs must belong to different subnets - which is exactly what you're attempting with eth0 (192.168.91.0/24) and eth0:0 (10.10.191.0/24).


Your current configuration looks technically correct, but let's examine some potential improvements:

# Original eth0 config
DEVICE=eth0
BOOTPROTO=static
IPADDR=192.168.91.250
NETMASK=255.255.255.0
GATEWAY=192.168.91.1  # Recommended to specify primary gateway
ONBOOT=yes

# Improved eth0:0 config
DEVICE=eth0:0
BOOTPROTO=static
IPADDR=10.10.191.210
NETMASK=255.255.255.0
# NETWORK and BROADCAST are optional in modern configs
ONPARENT=yes

The critical consideration is routing. While you can specify multiple gateways in interface configs, Linux will only use one default gateway (typically the one defined in the primary interface). For proper routing between subnets:
# View current routes
route -n

# Add specific route for secondary subnet
route add -net 10.10.191.0 netmask 255.255.255.0 gw 10.10.191.254 dev eth0:0

# Make persistent by adding to /etc/rc.local
echo "route add -net 10.10.191.0 netmask 255.255.255.0 gw 10.10.191.254 dev eth0:0" >> /etc/rc.local

Your /etc/resolv.conf should contain nameservers that can resolve for both networks. A typical configuration might look like:
nameserver 10.10.15.161
nameserver 10.10.18.36
nameserver 192.168.91.1  # Add primary network DNS if needed
options rotate timeout:1 attempts:2

After making changes, restart networking and verify:
service network restart

# Check interfaces
ifconfig -a

# Test connectivity
ping -I eth0 192.168.91.1
ping -I eth0:0 10.10.191.254

# Check routing
ip route show
traceroute 10.10.15.161

For more complex scenarios, consider these approaches:

Using policy-based routing for advanced multi-gateway setups
Implementing network namespaces for complete network isolation
Using bonding/teaming for redundancy and load balancing

Remember that interface aliasing is different from VLAN tagging (802.1Q) which creates logically separate networks on the same physical interface.