IP Addresses and ARP
|Document revision:||1.3 (Tue Sep 20 19:02:32 GMT 2005)|
The following Manual discusses IP address management and the Address Resolution Protocol settings. IP addresses serve as identification when communicating with other network devices using the TCP/IP protocol. In turn, communication between devices in one physical network proceeds with the help of Address Resolution Protocol and ARP addresses.
SpecificationsPackages required: system
License required: Level1
Submenu level: /ip address, /ip arp
Standards and Technologies: IP, ARP
Hardware usage: Not significant
IP AddressingSubmenu level: /ip address
IP addresses serve for a general host identification purposes in IP networks. Typical (IPv4) address consists of four octets. For proper addressing the router also needs the network mask value, id est which bits of the complete IP address refer to the address of the host, and which - to the address of the network. The network address value is calculated by binary AND operation from network mask and IP address values. It's also possible to specify IP address followed by slash "/" and amount of bits assigned to a network mask.
In most cases, it is enough to specify the address, the netmask, and the interface arguments. The network prefix and the broadcast address are calculated automatically.
It is possible to add multiple IP addresses to an interface or to leave the interface without any addresses assigned to it. Leaving a physical interface without an IP address is not a must when the bridging between interfaces is used. In case of bridging, the IP address can be assigned to any interface in the bridge, but actually the address will belong to the bridge interface. You can use /ip address print detail to see to which interface the address belongs to.
MikroTik RouterOS has following types of addresses:
- Static - manually assigned to the interface by a user
- Dynamic - automatically assigned to the interface by estabilished ppp, ppptp, or pppoe connections
Property Descriptionactual-interface (read-only: name) - only applicable to logical interfaces like bridges or tunnels. Holds the name of the actual hardware interface the logical one is bound to.address (IP address) - IP addressbroadcast (IP address; default: 255.255.255.255) - broadcasting IP address, calculated by default from an IP address and a network maskdisabled (yes | no; default: no) - specifies whether the address is disabled or notinterface (name) - interface name the IP address is assigned tonetmask (IP address; default: 0.0.0.0) - specifies network address part of an IP addressnetwork (IP address; default: 0.0.0.0) - IP address for the network. For point-to-point links it should be the address of the remote end
You cannot have two different IP addresses from the same network assigned to the router. Exempli gratia, the combination of IP address 10.0.0.1/24 on the ether1 interface and IP address 10.0.0.132/24 on the ether2 interface is invalid, because both addresses belong to the same network 10.0.0.0/24. Use addresses from different networks on different interfaces, or enable proxy-arp on ether1 or ether2.
[admin@MikroTik] ip address> add address=10.10.10.1/24 interface=ether2 [admin@MikroTik] ip address> print Flags: X - disabled, I - invalid, D - dynamic # ADDRESS NETWORK BROADCAST INTERFACE 0 188.8.131.52/24 184.108.40.206 220.127.116.11 ether2 1 10.5.7.244/24 10.5.7.0 10.5.7.255 ether1 2 10.10.10.1/24 10.10.10.0 10.10.10.255 ether2 [admin@MikroTik] ip address>
Address Resolution ProtocolSubmenu level: /ip arp
Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. Address Resolution Protocol is used to map OSI level 3 IP addreses to OSI level 2 MAC addreses. A router has a table of currently used ARP entries. Normally the table is built dynamically, but to increase network security, it can be built statically by means of adding static entries.
Property Descriptionaddress (IP address) - IP address to be mappedinterface (name) - interface name the IP address is assigned tomac-address (MAC address; default: 00:00:00:00:00:00) - MAC address to be mapped to
Maximal number of ARP entries is 8192.
If arp feature is turned off on the interface, i.e., arp=disabled is used, ARP requests from clients are not answered by the router. Therefore, static arp entry should be added to the clients as well. For example, the router's IP and MAC addresses should be added to the Windows workstations using the arp command:
C:\> arp -s 10.5.8.254 00-aa-00-62-c6-09
If arp property is set to reply-only on the interface, then router only replies to ARP requests. Neighbour MAC addresses will be resolved using /ip arp statically.
[admin@MikroTik] ip arp> add address=10.10.10.10 interface=ether2 mac-address=06 \ \... :21:00:56:00:12 [admin@MikroTik] ip arp> print Flags: X - disabled, I - invalid, H - DHCP, D - dynamic # ADDRESS MAC-ADDRESS INTERFACE 0 D 18.104.22.168 00:30:4F:1B:B3:D9 ether2 1 D 10.5.7.242 00:A0:24:9D:52:A4 ether1 2 10.10.10.10 06:21:00:56:00:12 ether2 [admin@MikroTik] ip arp>
If static arp entries are used for network security on an interface, you should set arp to 'reply-only' on that interface. Do it under the relevant /interface menu:
[admin@MikroTik] ip arp> /interface ethernet set ether2 arp=reply-only [admin@MikroTik] ip arp> print Flags: X - disabled, I - invalid, H - DHCP, D - dynamic # ADDRESS MAC-ADDRESS INTERFACE 0 D 10.5.7.242 00:A0:24:9D:52:A4 ether1 1 10.10.10.10 06:21:00:56:00:12 ether2 [admin@MikroTik] ip arp>
A router with properly configured proxy ARP feature acts like a transparent ARP proxy between directly connected networks. Consider the following network diagram:
Suppose the host A needs to communicate to host C. To do this, it needs to know host's C MAC address. As shown on the diagram above, host A has /24 network mask. That makes host A to believe that it is directly connected to the whole 192.168.0.0/24 network. When a computer needs to communicate to another one on a directly connected network, it sends a broadcast ARP request. Therefore host A sends a broadcast ARP request for the host C MAC address.
Broadcast ARP requests are sent to the broadcast MAC address FF:FF:FF:FF:FF:FF. Since the ARP request is a broadcast, it will reach all hosts in the network A, including the router R1, but it will not reach host C, because routers do not forward broadcasts by default. A router with enabled proxy ARP knows that the host C is on another subnet and will reply with its own MAC adress. The router with enabled proxy ARP always answer with its own MAC address if it has a route to the destination.
This behaviour can be usefull, for example, if you want to assign dial-in (ppp, pppoe, pptp) clients IP addresses from the same address space as used on the connected LAN.
Consider the following configuration:
The MikroTik Router setup is as follows:
admin@MikroTik] ip arp> /interface ethernet print Flags: X - disabled, R - running # NAME MTU MAC-ADDRESS ARP 0 R eth-LAN 1500 00:50:08:00:00:F5 proxy-arp [admin@MikroTik] ip arp> /interface print Flags: X - disabled, D - dynamic, R - running # NAME TYPE MTU 0 eth-LAN ether 1500 1 prism1 prism 1500 2 D pppoe-in25 pppoe-in 3 D pppoe-in26 pppoe-in [admin@MikroTik] ip arp> /ip address print Flags: X - disabled, I - invalid, D - dynamic # ADDRESS NETWORK BROADCAST INTERFACE 0 10.0.0.217/24 10.0.0.0 10.0.0.255 eth-LAN 1 D 10.0.0.217/32 10.0.0.230 0.0.0.0 pppoe-in25 2 D 10.0.0.217/32 10.0.0.231 0.0.0.0 pppoe-in26 [admin@MikroTik] ip arp> /ip route print Flags: X - disabled, I - invalid, D - dynamic, J - rejected, C - connect, S - static, R - rip, O - ospf, B - bgp # DST-ADDRESS G GATEWAY DISTANCE INTERFACE 0 S 0.0.0.0/0 r 10.0.0.1 1 eth-LAN 1 DC 10.0.0.0/24 r 0.0.0.0 0 eth-LAN 2 DC 10.0.0.230/32 r 0.0.0.0 0 pppoe-in25 3 DC 10.0.0.231/32 r 0.0.0.0 0 pppoe-in26 [admin@MikroTik] ip arp>
Unnumbered interfaces can be used on serial point-to-point links, e.g., MOXA or Cyclades interfaces. A private address should be put on the interface with the network being the same as the address on the router on the other side of the p2p link (there may be no IP on that interface, but there is an ip for that router).
[admin@MikroTik] ip address> add address=10.0.0.214/32 network=192.168.0.1 \ \... interface=pppsync [admin@MikroTik] ip address> print Flags: X - disabled, I - invalid, D - dynamic # ADDRESS NETWORK BROADCAST INTERFACE 0 10.0.0.214/32 192.168.0.1 192.168.0.1 pppsync [admin@MikroTik] ip address> [admin@MikroTik] ip address> .. route print detail Flags: X - disabled, I - invalid, D - dynamic, J - rejected, C - connect, S - static, R - rip, O - ospf, B - bgp 0 S dst-address=0.0.0.0/0 preferred-source=0.0.0.0 gateway=192.168.0.1 gateway-state=reachable distance=1 interface=pppsync 1 DC dst-address=192.168.0.1/32 preferred-source=10.0.0.214 gateway=0.0.0.0 gateway-state=reachable distance=0 interface=pppsync [admin@MikroTik] ip address>
As you can see, a dynamic connected route has been automatically added to the routes list. If you want the default gateway be the other router of the p2p link, just add a static route for it. It is shown as 0 in the example above.
Router shows that the IP address is invalid
Check whether the interface exists to which the IP address is assigned. Or maybe it is disabled. It is also possible that the system has crashed - reboot the router.
Router shows that the ARP entry is invalid
Check whether the interface exists to which the ARP entry is assigned. Or maybe it is disabled. Check also for an IP address for the particular interface.