 the QoS mechanisms and it is not tied with predefined bitrates as was the case with the leased lines in the past. Regarding security aspects, VPN provides encryption of user data by using IPsec, which provides encryption of the payload of the IP packet using security protocols, cryptographic algo- rithms, and keys. However, the IP packet header cannot be encrypted across the Internet simply because IP network routing is based on IP addresses placed in the packet header (for both IPv4 and IPv6). To provide security [2], IPsec uses the following two protocols: Authentication header (AH). This does source authentication, data integrity, but without encryption of the IP packets. The AH header is inserted between the IP header and payload fields, SO all intermediate routers process packets as usual. Encapsulating security payload (ESP). This provides confidentiality, host authentica- tion, and data integrity. The IP packet data field and ESP trailer are encrypted. IPsec protocols (autonomous system (AS) and ESP) are used for creation of VPNs, which in practice can be used in the following two modes: Transport mode. In this mode AH and ESP protocols provide protection for protocols above the IP (which is on the network layer, i.e. protocol layer 3). Tunnel mode. In this mode AH and ESP are used for tunneled packets, that is, IPsec encrypts the whole IP packet (including IP header and payload, where the data from higher layer protocols and user/application data are placed in the payload) and adds a new IP header to the encrypted original packet. The newly created packet is tunneled through the network. The tunnel starts at the node where the packet is encrypted and the new IP header is added. The tunnel ends at the network node where the packet will be decrypted and the original packet will be processed further (it is also possible to be placed in another tunnel for further transport over the IP network). The new header that is added to the IP packet has as the source address the IP address of a network interface (the 