Understanding ISP’s “4 Static IPs with 2 Usable” for Network Configuration and Subnetting


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When an ISP says they'll provide "4 static IPs with 2 usable", they're typically referring to a /30 subnet allocation. In IPv4 networking, this means:

Network Address: X.X.X.0
Usable Hosts:    X.X.X.1 and X.X.X.2
Broadcast:       X.X.X.3

Here's how you might configure this in a router (Cisco IOS example):

interface GigabitEthernet0/0
 ip address 203.0.113.1 255.255.255.252
 no shutdown

In this case:

  • 203.0.113.0 - Network address (reserved)
  • 203.0.113.1 - Your router's WAN interface
  • 203.0.113.2 - ISP's gateway
  • 203.0.113.3 - Broadcast address

This /30 subnet is the smallest usable network segment in IPv4. It's commonly used for:

  • Point-to-point connections
  • WAN links between routers
  • Leased line terminations

For a Linux server with multiple public IPs, you might configure additional addresses like this:

# Primary IP
ifconfig eth0 203.0.113.1 netmask 255.255.255.252

# Additional IP (if provided in a larger block)
ip addr add 203.0.113.5/29 dev eth0

With only 2 usable IPs, you'll likely need NAT for internal devices. Here's a basic iptables example:

iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to-source 203.0.113.1
iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

When testing your connection, remember:

ping 203.0.113.2  # Should respond (ISP gateway)
traceroute 8.8.8.8 # Verify routing path

When an ISP mentions "4 static IPs with 2 usable," they're referring to a /30 subnet allocation in IPv4 networking. Here's the technical breakdown:

Network ID:    First IP (reserved)
Usable Host 1: Second IP (assignable)
Usable Host 2: Third IP (assignable)
Broadcast:     Fourth IP (reserved)

In point-to-point connections (common with leased lines), ISPs typically allocate /30 subnets because:

  1. The ISP needs one IP for their router interface
  2. You need one IP for your router interface
  3. The remaining two IPs are reserved for network/broadcast addresses

Here's how to configure this in Cisco IOS:

interface GigabitEthernet0/0
 ip address 203.0.113.2 255.255.255.252
 description ISP Leased Line Connection

Assuming the ISP assigned 203.0.113.0/30:

  • 203.0.113.0 - Network address
  • 203.0.113.1 - ISP router
  • 203.0.113.2 - Your router
  • 203.0.113.3 - Broadcast address

For network architects planning NAT configurations:

ip nat pool PUBLIC_POOL 198.51.100.1 198.51.100.2 netmask 255.255.255.252
ip nat inside source list PRIVATE_NETS pool PUBLIC_POOL overload

Remember to account for:

  • Port forwarding requirements
  • Potential need for 1:1 NAT mappings
  • Future expansion constraints

If you need more usable IPs, consider:

# Requesting a /29 subnet (6 usable IPs) instead:
ipcalc 203.0.113.0/29
Network:   203.0.113.0/29
HostMin:   203.0.113.1
HostMax:   203.0.113.6
Broadcast: 203.0.113.7

Or implementing PAT (Port Address Translation) if you only need outgoing connectivity:

ip nat inside source list PRIVATE_NETS interface GigabitEthernet0/0 overload